Cost effective synthesis of oxide materials for lithium ion batteries

ABSTRACT

Methods for synthesizing crystalline Ni-rich cathode materials are disclosed. The Ni-rich cathode material may have a formula LiNixMnyMzCo1−x−y−zO2, where M represents one or more dopant metals, x≥0.6, 0.01≤y&lt;0.2, 0≤z≤0.05, and x+y+z≤1.0. The methods are cost-effective, and include methods for solid-state, molten-salt, and flash-sintering syntheses.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. application Ser. No. 16/951,868, filed Nov. 18, 2020, which claims the benefit of the earlier filing dates of U.S. Provisional Application No. 63/028,146, filed May 21, 2020, and U.S. Provisional Application No. 63/020,621, filed May 6, 2020, each of which is incorporated by reference in its entirety herein.

ACKNOWLEDGMENT OF GOVERNMENT SUPPORT

This invention was made with Government support under Contract DE-AC05-76RL01830 awarded by the U.S. Department of Energy. The Government has certain rights in the invention.

PARTIES TO JOINT RESEARCH AGREEMENT

The claimed invention was made as a result of activities undertaken within the scope of a Cooperative Research and Development Agreement between Battelle Memorial Institute and Albemarle Corporation.

FIELD

Methods of synthesizing crystalline oxide materials are disclosed, as well as cathodes including the crystalline oxide materials and lithium ion batteries including the cathodes.

SUMMARY

Embodiments of methods for synthesizing crystalline oxide materials are disclosed. Cathodes including the crystalline oxide materials and lithium ion batteries including the cathodes also are disclosed.

In some embodiments, lithium nickel manganese cobalt oxide is made by combining a molar excess of Li₂O with a solid oxide precursor comprising NixMn_(y)M_(z)Co_(1−x−y−z)O₂, where M represents one or more dopant metals, x≥0.6, 0.01≤y<0.2, 0≤z≤0.05, and x+y+z≤1.0. The solid oxide precursor and the Li₂O are heated in an oxygen atmosphere at a first temperature T₁ for a first effective period of time t₁ to produce a first product. The first product is cooled, and then subsequently heated in an oxygen atmosphere at a second temperature T₂ for a second effective period of time t₂ to produce lithium nickel manganese cobalt oxide having a formula LiNixMn_(y)M_(z)Co_(1−x−y−z)O₂. In some embodiments, the product is washed to remove any impurities before heating at the second temperature T₂. In some implementations, z is 0, and the lithium nickel manganese cobalt oxide has a formula LiNixMn_(y)Co_(1−x−y)O₂. In certain embodiments, the solid oxide precursor and the Li₂O are combined in a Li:solid oxide precursor molar ratio of 1.01:1 to 3:1.

In any of the foregoing or following embodiments, (i) the first temperature T₁ may be 800° C. to 1000° C., (ii) the first effective period of time t₁ may be 1 hour to 30 hours, or (iii) both (i) and (ii). In some embodiments, the first temperature T₁ is 900° C. to 1000° C., or 950° C. to 1000° C. In some implementations, the first effective period of time t₁ is 5 hours to 15 hours or 8 hours to 12 hours.

In any of the foregoing or following embodiments, (i) the second temperature T₂ may be 500° C. to 800° C., (ii) the second effective period of time t₂ may be 1 hour to 25 hours, or (iii) both (i) and (ii). In some embodiments, the second temperature T₂ is 550° C. to 650° C. In some implementations, the second effective period of time t₂ is 2 hours to 6 hours. In any of the foregoing embodiments, the method may further include preparing the solid oxide precursor by (i) preparing a 1.5-2.5 M solution comprising metal salts in water, the metal salts comprising a nickel (II) salt, a manganese (II) salt, a cobalt (II) salt, and optionally one or more dopant metal salts, wherein a mole fraction x of the nickel (II) salt in the solution is ≥0.6, a mole fraction y of the manganese (II) salt is 0.01≤y<0.2, a mole fraction z of the one or more dopant metal salts is 0≤z≤0.05, a mole fraction of the cobalt (II) salt is 1−x−y−z, and x+y+z≤1.0; (ii) combining the solution comprising metal salts in water with aqueous NH₃ and aqueous NaOH or KOH to provide a combined solution having a pH of 10.5-12 and a combined metal salt concentration of 0.1 M to 3 M; (iii) aging the combined solution for 1 hour to 48 hours at a temperature of 25° C. to 80° C. to co-precipitate hydroxides of nickel, manganese, and cobalt to provide a solid hydroxide precursor comprising NixMn_(y)M_(z)Co_(1−x−y−z)(OH)₂; and (iv) heating the solid hydroxide precursor comprising NixMn_(y)M_(z)Co_(1−x−y−z)(OH)₂ at a temperature of 500° C. to 1000° C. in an oxygen-containing atmosphere for 1 hour to 30 hours to convert the solid hydroxide precursor to the solid oxide precursor.

In some embodiments, a cathode comprises monocrystalline LiNixMn_(y)M_(z)Co_(1−x−y−z)O₂ wherein M represents one or more dopant metals; x≥0.6, 0.01≤y<0.2, z≤0.05, and x+y+z≤1.0; and a mean particle size of the monocrystalline LiNixMn_(y)M_(z)Co_(1−x−y−z)O₂ is 0.5 μm to 5 μm. In some embodiments, a battery system includes the cathode, an anode, an electrolyte, and a separator positioned between the anode and the cathode.

The foregoing and other objects, features, and advantages of the invention will become more apparent from the following detailed description, which proceeds with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.

FIG. 1 is a schematic diagram of one embodiment of a method for making a nickel manganese cobalt hydroxide precursor.

FIGS. 2A-2C are schematic diagrams of three exemplary embodiments of a solid-state method for making monocrystalline lithium nickel manganese cobalt oxide.

FIG. 3 is a schematic diagram of one embodiment of a molten salt method for making monocrystalline lithium nickel manganese cobalt oxide.

FIG. 4 is a schematic diagram of one embodiment of a flash-sintering method for making monocrystalline lithium nickel manganese cobalt oxide.

FIG. 5 is a schematic diagram of an exemplary lithium ion battery.

FIG. 6 is a schematic side elevation view of a simplified pouch cell.

FIGS. 7A-7H show characterization of single crystalline LiNi_(0.76)M_(0.14)C_(0.1)O₂ (NMC76). FIG. 7A is a scanning electron microscope (SEM) image of single crystalline NMC76. FIG. 7B is a cross-section image of single crystalline NMC76. FIG. 7C is a selected area electron diffraction (SAED) pattern of single crystalline NMC76. FIG. 7D shows synchrotron X-ray diffraction and Rietveld refinement patterns. FIG. 7E is a high-resolution HAADF-STEM (high-angular dark-field scanning transmission electron microscopy) image of single crystalline NMC76 (corresponding to the square in FIG. 7B). FIG. 7F is a higher-magnification image of the corresponding boxed region in FIG. 7E. FIG. 7G shows EDS (energy-dispersive x-ray spectroscopy) elemental mapping of Ni, Mn, Co, and O. FIG. 7H is an EDS overlapped image with line scanning showing the elemental distribution intensity.

FIGS. 8A-8F show characterization of a NMC76 secondary (polycrystalline) particle. FIG. 8A is a cross-section image. FIG. 8B shows SAED results. FIG. 8C is an HRTEM image of a primary particle. FIGS. 8D and 8E are HRTEM images of surface structure. FIG. 8F is an HRTEM image of a grain boundary.

FIG. 9 shows cyclic voltammetry curves of single crystalline NMC76 in different voltage windows using Li metal as the anode; scanning rate 0.1 mV/s.

FIG. 10 shows cycling stability of single crystalline NMC76 with different areal density between 2.7 and 4.5 V in half cells using Li metal as anode. Charge at 0.1C and discharge at 0.33C.

FIG. 11 shows initial charge-discharge curves between different cutoff voltages at 0.1C using Li metal as anode. Discharge capacity at 4.3, 4.4 and 4.5 V cutoff voltage are 184.9, 194.1 and 203.1 mAh/g, respectively.

FIGS. 12A-12C show electrochemical performance of single crystalline NMC76 at 4.2 V cutoff (12A), 4.3 V cutoff (12B), and 4.4 V cutoff (12C) tested in a full cell using graphite as the anode.

FIGS. 13A-13C show the corresponding charge-discharge curves of the cells in FIGS. 12A-12C, and accompanying SEM images of the single crystalline NMC76 after cycling.

FIGS. 14A-14B show an initial charge-discharge curve of single crystal NMC76 between 2.7-4.2 V (vs. graphite) (14A), and the middle voltage of the charge-discharge curves and the voltage difference over 200 cycles (14B).

FIGS. 15A-15B show an initial charge-discharge curve of single crystal NMC76 between 2.7-4.3 V (vs. graphite) (15A), and the middle voltage of the charge-discharge curves and the voltage difference over 200 cycles (15B).

FIGS. 16A-16B show an initial charge-discharge curve of single crystal NMC76 between 2.7-4.4 V (vs. graphite) (16A), and the middle voltage of the charge-discharge curves and the voltage difference over 200 cycles (16B).

FIGS. 17A-17F are SEM images of single crystal NMC76 after cycling tests: 2.7-4.2 V vs. graphite (17A-B); 2.7-4.3 V vs. graphite (17C-D); 2.7-4.4 V vs. graphite (17E-F).

FIGS. 18A-18L show the morphology and structure study of single crystalline NMC76. (18A) SEM image of single crystalline NMC76 after 200 cycles. (18B) Cross-section STEM bright field image of single crystalline NMC76 after 200 cycles. (18C) STEM bright field image of internal slicing. (18D) STEM-HAADF image around slicing area. Upper inset is zoomed image of gliding area. (18E) SAED of gliding area. (18F) EELS mapping of selected area in (18B). (18G) SEM images of single crystalline NMC76 initially charged to 4.8 V (vs. Li⁺/Li). (18H) SEM images of single crystalline NMC76 discharged to 2.7 V (after being charged to 4.8 V vs. Li⁺/Li). (18I-J) STEM images of single crystalline NMC76 at 4.4 V charge status (cycled in a full cell between 2.7-4.4 V for 120 cycles). (18K-L) STEM images of single crystalline NMC76 at discharge status (cycled in full cell between 2.7-4.4 V for 120 cycles).

FIG. 19 shows SEM images of single crystals on the same electrode before cycling. Eight different regions are randomly selected and analyzed in the SEM images 1-8. No pre-existing “gliding” lines are presenting in the pristine single crystalline NMC76.

FIG. 20 shows SEM images of single crystalline NMC76 located at eight different locations of the same electrode after 120 cycles (2.7-4.4 V vs. graphite). Gliding steps are obviously observed on cycled single crystals.

FIG. 21 shows SEM images of single crystalline NMC76 located at eight different locations of the same electrode after 200 cycles (between 2.7-4.4 V vs. graphite).

FIG. 22 shows electron energy loss spectra of O K-edge, Ni L-edge, Mn L-edge, and Co L-edge, EELS that correspond to test points 1-6 in FIG. 18B.

FIGS. 23A and 23B are SEM images of single crystalline NMC76 charged to 4.8 V (vs. Li+/Li) (23A), and discharged to 2.7 V after charging to 4.8 V (23B).

FIGS. 24A-24D are STEM images of single crystalline NMC76 after 120 cycles at charged status (cycled between 2.2-4.4 V vs. graphite). FIG. 24A is a cross-sectional image of the cycled single crystal. FIG. 24B is a STEM image of the boxed region in FIG. 24A. FIGS. 24C-24D are STEM images of the lower (24C) and upper (24D) tip regions around internal micro-cracks at the charged status.

FIGS. 25A-25E are STEM images of single crystalline NMC76 after 120 cycles at discharge state (cycled between 2.2-4.4 V vs. graphite). FIG. 25A is a cross-sectional image of the cycled single crystalline NMC76. FIG. 25B is a STEM image of the cycled single crystalline NMC76. FIG. 25C is a STEM bright field image of the boxed area of FIG. 25B. FIG. 25D is a STEM-HAADF image of the boxed area. FIG. 25E is a comparison of two selected areas from FIG. 25D.

FIGS. 26A-26F show surface structure and morphology evolution by in situ AFM and mechanical analysis for single crystalline NMC76. FIG. 26A is an AFM image at OCV state. FIGS. 26B-26C are a comparison of selected surface evolution during in situ AFM testing. FIG. 26D shows COMSOL simulated shear stress along the yz direction during charge (delithiation) at scaled time of 0.1T. FIG. 26E shows COMSOL simulated shear stress along the yz direction during discharge (lithiation) at scaled time of 0.1T. FIG. 26F is a schematic illustration of structural evolution of single crystalline NMC76 upon cycling.

FIGS. 27A and 27B are graphs showing gliding step width vs. testing time (FIG. 27A), and gliding step width vs. voltage (FIG. 27B).

FIGS. 28A-28D show the time evolution of Li concentration and stress during the de-lithiation process. FIG. 28A shows the Li concentration gradient in the de-lithiation process. FIGS. 28B-28D show radial stress (28B), tangential stress (28C), and axial stress (28D) distribution at different times.

FIGS. 29A-29D show the time evolution of Li concentration and stress during the lithiation process. FIG. 29A shows the Li concentration gradient in the lithiation process. FIGS. 29B-29D show radial stress (29B), tangential stress (29C), and axial stress (29D) distribution at different times.

FIGS. 30A-30C are SEM images and STEM images of single crystalline NMC76 showing microcracks, which propagate from the center to the surface, forming fractures.

FIGS. 31A-31D show the elastic stress tensor in the local Cartesian coordinate system on the yz plane at a scaled time of 0.1T solved numerically by the COMSOL model. FIG. 31A shows the normal stress along the zz direction, which will be responsible for crack opening along (003) direction, during delithiation (charging). FIG. 31B shows the shear stress along the yz direction, which is responsible for gliding, during delithiation (charging). FIG. 31C shows the shear stress along the yz direction during lithiation (discharging). FIG. 31D shows the shear stress along the yz direction during lithiation (discharging) with anisotropic strain.

FIG. 32 shows an SEM image of a 20 μm single crystal NMC76 and images obtained by in situ AFM.

FIGS. 33A-33D are SEM images of pristine Ni_(0.76)Mn_(0.14)Co_(0.1)(OH)₂ (33A), and oxides prepared by heating the Ni_(0.76)Mn_(0.14)Co_(0.1)(OH)₂ for 15 hours at 800° C. (33B), 900° C. (33C), or 1000° C. (33D).

FIG. 34 is a schematic diagram showing one embodiment of a molten-salt process as disclosed herein, as well as SEM images of the hydroxide precursor, the oxide precursor, and the single crystal LiNi_(0.76)Mn_(0.14)Co_(0.1)O₂.

FIGS. 35A and 35B are SEM images of Ni_(0.76)Mn_(0.14)Co_(0.1)O₂ prepared without NaCl (35A) and with NaCl (35B).

FIGS. 36A-36C show the initial charge-discharge curve of LiNi_(0.76)Mn_(0.14)Co_(0.1)O₂ prepared with and without NaCl (36A), the cycling stability of a single crystal NMC76 electrode (20 mg/cm²) in a full cell using graphite as the anode between 2.7-4.2V, charge at 0.1C and discharge at 0.33C (36B), and the cycling stability of a single crystal NMC76 electrode (21.5 mg/cm²) in a full cell using graphite as the anode between 2.7-4.3V. 1C=200 mA/g (36C).

FIGS. 37A-37C are SEM images of LiNi_(0.7)Mn_(0.22)Co_(0.08)O₂ washed with water (37A) and formamide (FM) (37B), and the initial charge-discharge curves of the samples (37C).

FIG. 38 is a schematic diagram comparing synthesis processes for polycrystalline and monocrystalline LiNixMn_(y)Co_(1−x−y)O₂.

FIGS. 39A-39C are SEM images of LiNi_(0.76)Mn_(0.14)Co_(0.1)O₂ prepared by flash sintering at ramping rates of 2° C./min (39A), 10° C./min (39B), and 20° C./min (39C).

FIGS. 40A-40B are SEM images of LiNi_(0.76)Mn_(0.14)Co_(0.1)O₂ prepared without preheating (40A) or with preheating (40B) prior to flash sintering.

FIG. 41 shows the charge-discharge curves of the flash-sintered LiNi_(0.76)Mn_(0.14)Co_(0.1)O₂ of FIGS. 40A and 40B.

FIGS. 42A-42C are SEM images of Ni_(0.76)Mn_(0.14)Co_(0.1)(OH)₂ (42A), oxide precursor (42B), and single crystal LiNi_(0.76)Mn_(0.14)Co_(0.1)O₂ (42C) prepared by a solid-state method as disclosed herein.

FIG. 43 shows charge-discharge curves of the LiNi_(0.76)Mn_(0.14)Co_(0.1)O₂ of FIG. 42C.

FIGS. 44A and 44B show the charge-discharge curves of a LiNi_(0.76)Mn_(0.14)Co_(0.1)O₂ cathode prepared by a molten-salt synthesis as disclosed herein (44A) and an SEM image of the LiNi_(0.76)Mn_(0.14)Co_(0.1)O₂ (44B).

FIGS. 45A and 45B show the charge-discharge curves of a LiNi_(0.76)Mn_(0.14)Co_(0.1)O₂ cathode prepared by a molten-salt synthesis as disclosed herein (45A) and an SEM image of the LiNi_(0.76)Mn_(0.14)Co_(0.1)O₂ (45B).

FIG. 46 shows SEM, HRTEM, and SAED images of single crystal LiNi_(0.76)Mn_(0.12)Co_(0.1)Mg_(0.01)Ti_(0.01)O₂.

FIGS. 47A and 47B are x-ray diffraction patterns comparing LiNi_(0.76)Mn_(0.14)Co_(0.1)O₂ and LiNi_(0.76)Mn_(0.12)Co_(0.1)Mg_(0.01)Ti_(0.01)O₂.

FIGS. 48A and 48B compare the charge-discharge curves (48A) and cycling stability (48B) of LiNi_(0.76)Mn_(0.14)Co_(0.1)O₂ and LiNi_(0.76)Mn_(0.12)Co_(0.1)Mg_(0.01)Ti_(0.01)O₂.

FIG. 49 is an SEM image of LiNi_(0.76)Mn_(0.12)Co_(0.1)Mg_(0.01)Ti_(0.01)O₂ after cycling.

FIG. 50 compares the charge and discharge capacities of NMC prepared from hydroxide precursors and from oxide precursors.

FIG. 51 is SEM images of two NMC products prepared from oxide precursors, wherein the product was milled and sieved after initial calcination (left image) or was not milled and sieved (right image).

FIG. 52 is SEM images of a transition metal oxide precursor (TMO), TMO mixed with unmilled Li₂O, and NMC products prepared at temperatures ranging from 750° C. to 950° C.; magnification 10,000×; scale bar 1 μm for all images except TMO-Li₂O with magnification 500×, scale bar 50 μm.

FIGS. 53A-53C are SEM images of NMC prepared from Li₂O at magnifications of 2,000× (53A, scale bar 10 μm), 5,000× (53B, scale bar 5 μm), and 11,000× (53C, scale bar 1 μm).

FIG. 54 compares the first cycle charge and discharge capacities of half cells comprising NMC cathodes prepared from hydroxide precursors and from oxide precursors.

FIG. 55 shows the performance of the cells of FIG. 53 over 20 cycles.

FIGS. 56A and 56B are photographs showing that a slurry comprising NMC made from LiOH undergoes gelation (56A), whereas a slurry comprising NMC made from Li₂O remains free flowing (56B).

DETAILED DESCRIPTION

Nickel-rich lithium-manganese-cobalt oxide (NMC) cathodes (LiNi_(x)Mn_(y)Co_(1−x−y)O₂) are promising cathodes for next-generation lithium ion batteries. Such batteries may be used, for example, for long-range electrical vehicles. In particular, NMC cathodes where x≥0.6, the capacity is ≥200 mAh/g, and the cathode is operable at high voltage (>3.8V) are desirable.

Traditionally, NMC cathodes are prepared by coprecipitation with aggregation of nano-sized primary particles into micro-sized secondary polycrystalline particles. This aggregated particle structure shortens the diffusion length of the primary particles and increases the number of pores and grain boundaries within the secondary particles, which accelerate the electrochemical reaction and improves the rate capability of NMC. Secondary micron-sized particles formed of agglomerated nano-sized primary particles are the most common morphology for conventional NMC cathodes. However, as the Ni content increases above 0.6, challenges arise. For example, such Ni-rich NMC cathodes are subject to moisture sensitivity, aggressive side reactions, and/or gas generation during cycling, raising safety concerns. These challenges are attributable to the large surface area of the secondary particles. Additionally, while creating spherical-secondary polycrystalline NMC particles reduces the surface/volume ratio, pulverization along the weak internal grain boundaries is generally observed after cycling. These cracks are induced by the non-uniform volume change of primary particles during cycling and exacerbated by the anisotropy among individual particles and grains in the polycrystalline NMC. The intergranular cracking exposes new surfaces to electrolyte for side reactions, which accelerates cell degradation. As the Ni content becomes ≥0.8 in NMC, the major challenge in Ni-rich NMC cathodes becomes quite different from those in conventional NMC. For example, NMC811 is very sensitive to moisture, which creates challenges for manufacturing, storing and transporting the Ni-rich NMC. After extensive cycling gas generation by the side reactions raises safety concerns.

This disclosure concerns embodiments of methods for synthesizing single crystalline Ni-rich cathode materials. Some embodiments of the disclosed methods may be used for synthesizing large batches, e.g., 1 kg or more, of single crystal lithium nickel manganese cobalt oxide. In some embodiments, the single crystalline Ni-rich cathodes comprise monocrystalline lithium nickel manganese cobalt oxide. In certain embodiments, single crystalline cathodes include reduced surface areas, phase boundaries, and/or more integrated crystal structures compared to polycrystalline cathodes. Advantageously some embodiments of the single crystalline Ni-rich cathodes demonstrate reduced gassing and/or particle cracking along grain boundaries during cycling. In some embodiments, the monocrystalline lithium nickel manganese cobalt oxide has a formula LiNixMn_(y)M_(z)Co_(1−x−y−z)O₂, where M represents one or more dopant metals, x≥0.6, 0.02≤y<0.2, 0≤z≤0.05, and x+y+z≤1.0. In certain embodiments, the formula is LiNixMn_(y)Co_(1−x−y)O₂ where x≥0.6, 0.02≤y<0.2, and x+y≤1.0.

I. Definitions and Abbreviations

The following explanations of terms and abbreviations are provided to better describe the present disclosure and to guide those of ordinary skill in the art in the practice of the present disclosure. As used herein, “comprising” means “including” and the singular forms “a” or “an” or “the” include plural references unless the context clearly dictates otherwise. The term “or” refers to a single element of stated alternative elements or a combination of two or more elements, unless the context clearly indicates otherwise.

Unless explained otherwise, all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which this disclosure belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present disclosure, suitable methods and materials are described below. The materials, methods, and examples are illustrative only and not intended to be limiting. Other features of the disclosure are apparent from the following detailed description and the claims.

The disclosure of numerical ranges should be understood as referring to each discrete point within the range, inclusive of endpoints, unless otherwise noted. Unless otherwise indicated, all numbers expressing quantities of components, molecular weights, percentages, temperatures, times, and so forth, as used in the specification or claims are to be understood as being modified by the term “about.” Accordingly, unless otherwise implicitly or explicitly indicated, or unless the context is properly understood by a person of ordinary skill in the art to have a more definitive construction, the numerical parameters set forth are approximations that may depend on the desired properties sought and/or limits of detection under standard test conditions/methods as known to those of ordinary skill in the art. When directly and explicitly distinguishing embodiments from discussed prior art, the embodiment numbers are not approximates unless the word “about” is recited.

Although there are alternatives for various components, parameters, operating conditions, etc. set forth herein, that does not mean that those alternatives are necessarily equivalent and/or perform equally well. Nor does it mean that the alternatives are listed in a preferred order unless stated otherwise.

Definitions of common terms in chemistry may be found in Richard J. Lewis, Sr. (ed.), Hawley's Condensed Chemical Dictionary, published by John Wiley & Sons, Inc., 2016 (ISBN 978-1-118-13515-0).

In order to facilitate review of the various embodiments of the disclosure, the following explanations of specific terms are provided:

Active salt: As used herein, the term “active salt” refers to a salt that significantly participates in electrochemical processes of electrochemical devices. In the case of batteries, it refers to charge and discharge processes contributing to the energy conversions that ultimately enable the battery to deliver/store energy. As used herein, the term “active salt” refers to a salt that constitutes at least 5% of the redox active materials participating in redox reactions during battery cycling after initial charging.

Annealing: A process in which a material is heated to a specified temperature for a specified period of time and then gradually cooled. The annealing process may remove internal strains from previous operations, and can eliminate distortions and imperfections to produce a stronger and more uniform material.

Anode: An electrode through which electric charge flows into a polarized electrical device. From an electrochemical point of view, negatively-charged anions move toward the anode and/or positively-charged cations move away from it to balance the electrons leaving via external circuitry.

Areal capacity or specific areal capacity is the capacity per unit area of the electrode (or active material) surface, and is typically expressed in united of mAh cm⁻².

Capacity: The capacity of a battery is the amount of electrical charge a battery can deliver. The capacity is typically expressed in units of mAh, or Ah, and indicates the maximum constant current a battery can produce over a period of one hour. For example, a battery with a capacity of 100 mAh can deliver a current of 100 mA for one hour or a current of 5 mA for 20 hours.

Cathode: An electrode through which electric charge flows out of a polarized electrical device. From an electrochemical point of view, positively charged cations invariably move toward the cathode and/or negatively charged anions move away from it to balance the electrons arriving from external circuitry.

Cell: As used herein, a cell refers to an electrochemical device used for generating a voltage or current from a chemical reaction, or the reverse in which a chemical reaction is induced by a current. Examples include voltaic cells, electrolytic cells, and fuel cells, among others. A battery includes one or more cells. The terms “cell” and “battery” are used interchangeably when referring to a battery containing only one cell.

Coulombic efficiency (CE): The efficiency with which charges are transferred in a system facilitating an electrochemical reaction. CE may be defined as the amount of charge exiting the battery during the discharge cycle divided by the amount of charge entering the battery during the charging cycle.

Current density: A term referring to the amount of current per unit area. Current density is typically expressed in units of mA/cm².

Electrolyte: A substance containing free ions that behaves as an electrically conductive medium. Electrolytes generally comprise ions in a solution, but molten electrolytes and solid electrolytes also are known.

Gel: A colloidal system comprising a solid three-dimensional network within a liquid. By weight, a gel is primarily liquid, but behaves like a solid due to a three-dimensional network of entangled and/or crosslinked molecules of a solid within the liquid.

Gelation: The point at which a slurry becomes a colloidal system that behaves like a solid. Gelation can be determined visually, i.e., by observing that the slurry behaves like a solid and cannot be poured.

Microparticle: As used herein, the term “microparticle” refers to a particle with a size measured in microns, such as a particle with a diameter of 1-100 μm.

Nanoparticle: As used herein, the term “nanoparticle” particle that has a size measured in nanometers, such as a particle with a diameter of 1-100 nm.

Pouch cell: A pouch cell is a battery completely, or substantially completely, encased in a flexible outer covering, e.g., a heat-sealable foil, a fabric, or a polymer membrane. The term “flexible” means that the outer covering is easy to bend without breaking; accordingly, the outer covering can be wrapped around the battery components. Because a pouch cell lacks an outer hard shell, it is flexible and weighs less than conventional batteries.

Precursor: A precursor participates in a chemical reaction to form another compound. As used herein, the term “precursor” refers to metal-containing compounds used to prepare lithium nickel manganese cobalt oxide and metal-doped lithium nickel manganese cobalt oxide. Separator: A battery separator is a porous sheet or film placed between the anode and cathode. It prevents physical contact between the anode and cathode while facilitating ionic transport.

Slurry: A liquid-solid fluid mixture with a specific gravity greater than 1.

Solid state: Composed of solid components. As defined herein, a solid-state synthesis proceeds with solid components directly without using sintering agents.

Specific capacity: A term that refers to capacity per unit of mass. Specific capacity may be expressed in units of mAh/g, and often is expressed as mAh/g carbon when referring to a carbon-based electrode in Li/air batteries.

Specific energy: A term that refers to energy per unit of mass. Specific energy is commonly expressed in units of Wh/kg or J/kg.

II. Synthesis of Crystalline Oxide Materials

Embodiments of methods for making monocrystalline lithium nickel manganese cobalt oxide (NMC) and metal-doped lithium nickel manganese cobalt oxide are disclosed. In some embodiments, the monocrystalline lithium nickel manganese cobalt oxide has a formula LiNixMn_(y)M_(z)Co_(1−x−y−z)O₂, where M represents one or more dopant metals, x≥0.6, 0.02≤y<0.2, 0≤z≤0.05, and x+y+z≤1.0. More particularly, 0.62≤x+y+z≤1.0. In certain embodiments, z is 0, and the monocrystalline lithium nickel manganese cobalt oxide has a formula LiNixMn_(y)Co_(1−x−y)O₂, where x≥0.6, 0.02≤y<0.2, and x+y≤1.0. More particularly, 0.62≤x+y≤1.0. In some embodiments, x=0.65-0.99, y=0.01-0.2, z=0-0.02, and x+y+z=0.66-1.0. In certain embodiments, x=0.65-0.9, y=0.05-0.2, z=0-0.02, and x+y+z=0.7-0.95. In some examples, x is 0.7-0.9, such as 0.75-0.9 or 0.8-0.9; y is 0.05-0.15, such as 0.05-0.14 or 0.05-0.1; z is 0-0.02; and x+y+z is 0.8-0.98, such as 0.8-0.95.

When the monocrystalline lithium nickel manganese cobalt oxide is doped, the general formula is LiNixMn_(y)M_(z)Co_(1−x−y−z)O₂, where M represents one or more dopant metals. In some embodiments, M represents two or more dopant metals. Thus, M_(z) may refer collectively to M1_(z1)+M2_(z2)+M3_(z3) . . . +MP_(zp), where M1, M2, M3, etc. represent the dopant metals, and z1+z2+z3 . . . +zp=z. Suitable dopant metals include, but are not limited to, Mg, Ti, Al, Zn, Fe (e.g., Fe³⁺), Zr, Sn (e.g., Sn⁴⁺), Sc, V, Cr, Fe, Cu, Zu, Ga, Y, Zr, Nb, Mo, Ru, Ta, W, Ir, and combinations thereof.

The lithium nickel manganese cobalt oxide crystals prepared by embodiments of the disclosed methods are microparticles. In some embodiments, the single crystals have a mean particle size of 1-5 μm, such as 1˜4 μm or 1-3 μm. This feature is in stark contrast to traditional NMC comprising primary nanoparticles particles aggregated into secondary polycrystalline microparticles.

A. Hydroxide Precursor Synthesis

In any the foregoing or following embodiments, the synthesis may begin with solid precursors comprising hydroxides of nickel, manganese, and cobalt. In some embodiments, the synthesis may further include solid hydroxide precursors of one or more dopant metals, e.g., hydroxides of Mg, Ti, Al, Zn, Fe, Zr, Sn, or any combination thereof. In some embodiments, the hydroxide precursor comprises NixMn_(y)M_(z)Co_(1−x−y−z)(OH)₂, where M represents one or more dopant metals, x≥0.6, 0.01≤y<0.2, z≤0.05, and x+y+z≤1.0. More particularly, 0.62≤x+y+z≤1.0. In some embodiments, x=0.65-0.99, y=0.01-0.2, z=0-0.02, and x+y+z=0.66-1.0. In an independent embodiment, x=0.65-0.95, y=0.01-0.2, z=0-0.02, and x+y+z=0.66-0.98. In another independent embodiment, x=0.65-0.9, y=0.05-0.2, z=0-0.02, and x+y+z=0.7-0.95. In some examples, x is 0.7-0.9, such as 0.75-0.9 or 0.8-0.9; y is 0.05-0.15, such as 0.05-0.14 or 0.05-0.1; z is 0-0.02; and x+y+z is 0.8-0.98, such as 0.8-0.95.

In any the foregoing or following embodiments, the method of synthesizing monocrystalline lithium nickel manganese cobalt oxide (including doped variants), may include synthesizing the hydroxide precursors. In some embodiments (FIG. 1), the hydroxide precursors are synthesized by preparing a 1.5 M to 2.5 M solution comprising metal salts in water (101), the metal salts comprising a nickel salt, a manganese salt, a cobalt salt, and optionally one or more dopant metal salts; combining the solution comprising metal salts in water with aqueous NH₃ and aqueous NaOH or KOH to provide a combined solution having a pH of 10.5-12 (102), aging the solution for 5-48 hours to co-precipitate hydroxides of nickel, manganese; and cobalt to provide the solid hydroxide precursor (103); and drying the solid hydroxide precursor (104). In any of the foregoing or following embodiments, the metal salts may include a nickel (II) salt, a manganese (II) salt, and a cobalt (II) salt (if x+y+z<1).

The solution comprising metal salts in water has a concentration of 1.5-2.5 M, wherein 1.5 M to 2.5 M is a total concentration of all salts in the water. In some embodiments, the concentration is 1.7 M to 2.3 M, 1.8 M to 2.2 M, or 1.9 M to 2.1 M. In any of the foregoing or following embodiments, the salts may be sulfates, nitrates, chlorides, acetates, or a combination thereof. In one embodiment, the salts are sulfates. In another embodiment, the salts are nitrates. The concentration of each metal salt is selected based on a desired amount of the metal in the final product. For example, when a hydroxide precursor comprising Ni_(0.76)Mn_(0.14)Co_(0.1)(OH)₂ is prepared, nickel, manganese, and cobalt salts are combined in a Ni:Mn:Co molar ratio of 0.76:0.14:0.1. Similarly, if a hydroxide precursor comprising Ni_(0.76)Mn_(0.12)Co_(0.1)Mg_(0.01)Ti_(0.01)(OH)₂ is prepared, nickel, manganese, cobalt, magnesium, and titanium salts are combined in a Ni:Mn:Co:Mg:Ti molar ratio of 0.76:0.12:0.01:0.01.

In any of the foregoing or following embodiments, combining the solution comprising metal salts in water with aqueous NH₃ and aqueous NaOH or KOH to provide a pH of 10.5-12 may comprise preparing an aqueous NH₃ solution comprising 0.5 wt % to 1 wt % or 0.2-0.5 M NH₃ in water; preheating the aqueous NH₃ solution to 25° C. to 80° C.; adding the metal salt solution, additional concentrated aqueous ammonia (e.g., 25 wt % to 35 wt % or 13 M to 18 M NH₃.H₂O), and aqueous NaOH or KOH to provide a pH of 10.5-12 and a final metal salt concentration of 0.1 M to 3 M. In some embodiments, the aqueous NH₃ solution is preheated to 30° C. to 75° C., such as 35° C. to 70° C., 40° C. to 60° C., or 45° C. to 55° C. In some embodiments, the final metal salt concentration is 0.1 M to 2 M, 0.1 M to 1 M, or 0.2 M to 0.8 M. In some embodiments, the metal salt solution and additional concentrated ammonia are added at the same time at a volumetric ratio 2-4 parts metal salt solution to one part concentrated ammonia solution. The aqueous NH₃ solution may be stirred continuously as the metal salt solution and concentrated ammonia solution are added. In certain examples, the metal salt solution is added at a rate of 3 mL/minute, and the concentrated ammonia is added at a rate of 1 mL/minute. Sufficient aqueous NaOH or KOH is added to provide the combined solution with a pH of 10.5-12, such as a pH of 11-11.5. In some embodiments, the aqueous NaOH or KOH has a concentration of 6 M to 10 M. In any of the foregoing or following embodiments, the combined solution may be aged for 5-48 hours at a temperature of 25° C. to 80° C. to co-precipitate hydroxides of the metals (Ni, Mn, Co, and any dopant metals), thereby producing the hydroxide precursor. In some embodiments, the combined solution is stirred continuously while aging. In certain embodiments, the combined solution is aged for 10 hours to 48 hours, 15 hours to 45 hours, 20 hours to 40 hours, or 25 hours to 35 hours. In some embodiments, the temperature is 30° C. to 75° C., such as 35° C. to 70° C., 40° C. to 60° C., or 45° C. to 55° C. In some examples, the combined solution was aged for 30 hours at 50° C. with continuous stirring.

In any of the foregoing or following embodiments, the hydroxide precursor may be collected by any suitable method. In some embodiments, the aged combined solution is filtered to collect the co-precipitated hydroxides. The collected hydroxide precursor may be washed, e.g., with deionized water, to remove impurities, such as ammonia, residual NaOH or KOH, and/or soluble sulfate and/or nitrate salts. The hydroxide precursor is then dried. In some embodiments, the hydroxide precursor is dried at a temperature of 80° C. to 120° C., such as 90° C. to 110° C., for a period of 5 hours to 20 hours, such as 10 hours to 15 hours.

Advantageously, the low concentration, 1.5 M to 2.5 M, of the metal salt solution facilitates formation of small hydroxide precursor particles. In any of the foregoing embodiments, embodiments, the hydroxide precursor particles may have a mean size of 0.5 μm to 10 μm. In some embodiments, the hydroxide precursor particles have a mean size of 0.5 μm to 7.5 μm, 0.5 μm to 5 μm, or 0.5 μm to 2.5 μm.

B. Solid-State Method

Monocrystalline or polycrystalline lithium nickel manganese cobalt oxide (or a doped variant thereof) may be synthesized by a solid-state method. With reference to FIG. 2A, in some embodiments, the solid-state method comprises heating a solid hydroxide precursor comprising NixMn_(y)M_(z)Co_(1−x−y−z)(OH)₂ at a temperature T_(S1) in an oxygen-containing atmosphere for an effective period of time t₁ to convert the solid hydroxide precursor to a solid oxide precursor (201 a); combining the solid oxide precursor with a molar excess of a lithium compound (202 a); heating the solid oxide precursor and the lithium compound at a temperature T_(S2) for an effective period of time t₂ to produce a first product (203 a); cooling the first product to ambient temperature (204 a); reducing a mean particle size of the first product to 0.1 μm to 10 μm (205 a); heating the first product having the reduced mean particle size at a temperature T_(S3) for an effective period of time t₃ to produce a second product (206 a); cooling the second product to ambient temperature (207 a); reducing a mean particle size of the second product to 0.1 μm to 10 μm (208 a); and heating the second product having the reduced mean particle size at a temperature T_(S4) for an effective period of time t₄ to produce monocrystalline lithium nickel manganese cobalt oxide having a formula LiNixMn_(y)M_(z)Co_(1−x−y−z)O₂ (209 a). In some embodiments (FIG. 2B), the process further comprises cooling the LiNixMn_(y)M_(z)Co_(1−x−y−z)O₂ to ambient temperature (210 b), reducing a mean particle size of the LiNixMn_(y)M_(z)Co_(1−x−y−z)O₂ to provide milled LiNixMn_(y)M_(z)Co_(1−x−y−z)O₂ (211 b), washing the milled LiNixMn_(y)M_(z)Co_(1−x−y−z)O₂ (e.g., with deionized water) to remove any impurities (212 b) and provide a washed product, and heating the washed product at a temperature T_(S5) in an oxygen-containing atmosphere for an effective period of time t₅ (213 b) to restore any lost oxygen in the crystal lattices. In some implementations, providing the washed product further comprises heating the product (e.g., at 60° C. to 120° C., 70° C. to 120° C., 60° C. to 95° C., or 70° C. to 90° C.) in a vacuum for an effective period of time (e.g., 1-3 hours) to remove any water before heating the washed product in the oxygen-containing atmosphere to restore lost oxygen.

The inventors surprisingly discovered that the process could be reduced to just five steps, as shown in FIG. 2C, when the starting materials are lithium oxide and the solid oxide precursor. The steps in FIG. 2C are numbered according to the corresponding steps in FIG. 2B. Advantageously, the four heating steps shown in FIG. 2B are reduced to just two heating steps, and the intermediate milling and sieving steps shown in the exemplary process of FIG. 2B are eliminated. Because Li₂O does not melt during the process, reactant/product caking does not occur and milling/sieving is no longer required. In a first step (202 c) of the streamlined process, the solid oxide precursor is combined with a molar excess of Li₂O to form an oxide mixture. The oxide mixture is heated in an oxygen-containing atmosphere at a temperature T_(S2) for an effective period of time t₂ to produce a product comprising LiNixMn_(y)M_(z)Co_(1−x−y−z)O₂ (203 c). The product is then cooled, e.g., to ambient temperature (210 c). Optionally, the product is washed (e.g., with deionized water) to remove any impurities (212 c). In some implementations, such as when the Li:solid oxide precursor molar ratio is low (e.g., 1.03:1), washing is unnecessary. The product is heated at a temperature T_(S5) in an oxygen-containing atmosphere for an effective period of time t₅ (213 c) to restore any lost oxygen in the crystal lattices. In some implementations, providing the washed product further comprises heating the product (e.g., at 60° C. to 120° C., 70° C. to 120° C., 60° C. to 95° C., or 70° C. to 90° C.) in a vacuum for an effective period of time (e.g., 1-3 hours) to remove any water before heating the washed product in the oxygen-containing atmosphere to restore lost oxygen. Advantageously, the streamlined method exemplified in FIG. 2C provides significant energy, time, and cost savings compared to the methods exemplified in FIGS. 2A and 2B, while providing a final product of similar quality and efficacy as demonstrated in the examples below. The final product yield is also increased due to fewer processing steps. Additionally, starting with lithium oxide and a solid oxide precursor eliminates water generation in the initial steps. The water is caustic and corrodes the manufacturing equipment. Eliminating the water generation thus reduces corrosion of the manufacturing equipment, with accompanying reductions in maintenance downtime and costs. In some examples, LiNixMn_(y)M_(z)Co_(1−x−y−z)O₂ produced by the method of FIG. 2C exhibits better slurry properties, compared to LiNixMn_(y)M_(z)Co_(1−x−y−z)O₂ produced by other methods, and does not undergo gelation when used in electrode preparation.

In the foregoing formulas, M represents one or more dopant metals, x≥0.6, 0.01≤y<0.2, z≤0.05, and x+y+z≤1.0. More particularly, 0.62≤x+y+z≤1.0. In some embodiments, x=0.65-0.99, y=0.01-0.2, z=0-0.02, and x+y+z=0.7-1.0. In an independent embodiment, x=0.65-0.95, y=0.01-0.2, z=0-0.02, and x+y+z=0.7-0.98. In another independent embodiment, x=0.65-0.9, y=0.05-0.2, z=0-0.02, and x+y+z=0.7-0.95. In some examples, x=0.7-0.9, y=0.05-0.15, z=0-0.02, and x+y+z=0.7-0.95. In one embodiment, x is 0.76, y is 0.14, and z is 0. In an independent embodiment, x is 0.76, y is 0.12, and z is 0.02. In another independent embodiment, x is 0.8, y is 0.1, and z is 0. In still another independent embodiment, x is 0.9, y is 0.05, and z is 0.

In any of the foregoing or following embodiments, the temperature T_(S1) may be 400° C. to 1000° C. and/or the effective period of time t₁ may be 1 hour to 30 hours. In some embodiments, the temperature T_(S1) is 500° C. to 1000° C., 600° C. to 1000° C., 800° C. to 1000° C., or 850° C. to 950° C. In certain examples, the temperature T_(S1) was 900° C. Advantageously, the temperature T_(S1) is below a melting point of the hydroxide precursor. In any of the foregoing or following embodiments, the temperature may be increased to the temperature T_(S1) at a ramping rate of 1° C./minute to 300° C./minute, such as a ramping rate of 1° C./minute to 200° C./minute, 1° C./minute to 100° C./minute, 1° C./minute to 50° C./minute, 5° C./minute to 25° C./minute, or 5° C./minute to 15° C./minute. In one example, the ramping rate was 10° C./minute. The temperature T_(S1) is then maintained for the effective period of time t₁. In some embodiments, the effective period of time t₁ is 5 hours to 25 hours, 10 hours to 20 hours, or 12 hours to 18 hours. In certain examples, the effective period of time t₁ was 15 hours. In any of the foregoing or following embodiments, the oxygen-containing atmosphere may be pure oxygen or air. As used herein, “pure oxygen” means at least 95 mol % oxygen. In any of the foregoing or following embodiments, at a majority or all of the solid hydroxide precursor may be converted to the solid oxide precursor. In some embodiments, 90 wt % to 100 wt %, such as 95 wt % to 100 wt %, 97 wt % to 100 wt %, 98 wt % to 100 wt %, or 99 wt % to 100 wt % of the solid hydroxide precursor is converted to the solid oxide precursor. In certain embodiments, all of the solid hydroxide precursor is converted to the solid oxide precursor.

The solid oxide precursor is combined with a molar excess of a Li compound. In any of the foregoing or following embodiments, the Li compound may comprise lithium hydroxide, lithium carbonate, lithium nitrate, lithium oxide, lithium peroxide, lithium acetate, lithium oxalate or any combination thereof. In some embodiments of the processes exemplified in FIGS. 2A and 2B, the Li compound comprises lithium hydroxide. The LiOH may be anhydrous or a hydrated salt, e.g., LiOH.H₂O. In the embodiment shown in FIG. 2C, the Li compound comprises or consists of Li₂O. In any of the foregoing or following embodiments, the Li compound may have a mean particle size of 10 μm to 150 μm, such as 10 μm to 100 μm. In any of the foregoing or following embodiments, the solid oxide precursor and the lithium compound may be combined in a Li:solid oxide precursor molar ratio of 0.8:1 to 3:1, such as a molar ratio of 0.9:1 to 3:1, 1:1 to 3:1, 1.01:1 to 3:1, 1.03:1 to 3:1, 1.05:1 to 3:1, 1:05:1 to 2:1, 1:05:1 to 1.5:1, 1.1:1 to 1.4:1 or 1.1:1 to 1.2:1. The mixture of solid oxide precursor and the Li compound is subjected to a series of three annealing processes to form a first product, a second product, and the LiNixMn_(y)M_(z)Co_(1−x−y−z)O₂.

The mixture of solid oxide precursor and the Li compound is heated at a temperature T_(S2) for an effective period of time t₂ to produce a first product. In any of the foregoing or following embodiments, the temperature T_(S2) may be 400° C. to 1000° C. and/or the effective period of time t₂ may be 1 hour to 30 hours. In some embodiments, the temperature T_(S2) is 400° C. to 800° C., 400° C. to 600° C., or 450° C. to 550° C. In some alternative embodiments, the temperature T_(S2) is 800° C. to 1000° C., such as 900° C. to 1000° C. Advantageously the temperature T_(S2) is less than a melting or vaporization temperature of the oxide precursor and lithium compound. In some examples, the temperature T_(S2) was 500° C. In other examples, the temperature T_(S2) was 800° C., 850° C., 900° C., 925° C., or 950° C. In some embodiments, the effective period of time t₂ is 1 hour to 5 hours, 1 hour to 20 hours, 1 hour to 12 hours, 1 hour to 10 hours, 2 hours to 6 hours, or 8 hours to 12 hours. In some examples, the effective period of time t₂ was 5 hours. In other examples, the effective period of time was 10 hours.

When the streamlined process including a single initial heating step, e.g., as exemplified in FIG. 2C is utilized, the temperature T_(S2) may be 800° C. to 1000° C., and the effective period of time t₂ may be 8 hours to 12 hours. In some implementations, the temperature T_(S2) is 800° C. to 900° C., and the product is polycrystalline or a mixture of monocrystalline and polycrystalline LiNixMn_(y)M_(z)Co_(1−x−y−z)O₂. At temperatures of 900° C. or greater, the proportion of monocrystalline LiNixMn_(y)M_(z)Co_(1−x−y−z)O₂ increases. At 950° C. or higher, most particles are individual micron-sized single crystals without aggregation and with few or no secondary particles. In certain embodiments, the temperature T_(S2) is 800° C. to 950° C.

In the process exemplified in FIG. 2B, the first product is cooled to ambient temperature and a mean particle size of the first product is reduced to 0.1 μm to 10 μm (204 b, 205 b). In some embodiments, the mean particle size is reduced to 0.2 μm to 10 μm, 0.5 μm to 10 μm, or 1 μm to 10 μm. In any of the foregoing or following embodiments, cooling the first product to ambient temperature may comprise cooling the first product to 20° C. to 30° C., such as to 20° C. to 25° C. In any of the foregoing or following embodiments, reducing a mean particle size of the first product to 0.1 μm to 10 μm may comprise grinding or milling the first product to achieve the desired particle size.

In the exemplary process of FIG. 2B, the first product having the reduced mean particle size is heated at a temperature T_(S3) for an effective period of time t₃ to produce a second product (206 b). In any of the foregoing or following embodiments, the temperature T_(S3) may be 600° C. to 1000° C. and/or the effective period of time t₃ may be 1-30 hours. In some embodiments, the temperature T_(S3) is 700° C. to 1000° C., 700° C. to 900° C., or 750° C. to 850° C. In certain examples, the temperature T_(S3) was 800° C. In some embodiments, the effective period of time t₃ is 1-25 hours, 1-20 hours, 1-10 hours, or 2-6 hours. In certain examples, the effective period of time t₃ was 5 hours.

In the exemplary process of FIG. 2B, the second product is cooled to ambient temperature and a mean particle size of the second product is reduced to 0.1 μm to 10 μm (207 b, 208 b). In some embodiments, the mean particle size is reduced to 0.2 μm to 10 μm, 0.5 μm to 10 μm, or 1 μm to 10 μm. In any of the foregoing or following embodiments, cooling the second product to ambient temperature may comprise cooling the second product to 20° C. to 30° C., such as to 20° C. to 25° C. In any of the foregoing or following embodiments, reducing a mean particle size of the second product to 0.1 μm to 10 μm may comprise grinding or milling the second product to achieve the desired particle size.

In the exemplary process of FIG. 2B, the second product having the reduced mean particle size is heated at a temperature T_(S4) for an effective period of time t₄ to produce monocrystalline lithium nickel manganese cobalt oxide having a formula LiNixMn_(y)M_(z)Co_(1−x−y−z)O₂ (209 b). In any of the foregoing or following embodiments, the temperature T_(S4) may be 500° C. to 1000° C. and/or the effective period of time t₄ may be 1-30 hours. In some embodiments, the temperature T_(S4) is 600° C. to 1000° C., 700° C. to 1000° C., 700° C. to 900° C., or 750° C. to 850° C. In certain examples, the temperature T_(S4) was 800° C. In some embodiments, the effective period of time t₄ is 1 hour to 25 hours, 1 hour to 20 hours, 1 hour to 10 hours, or 2 hours to 6 hours. In certain examples, the effective period of time t₄ was 5 hours.

In some embodiments, e.g., as shown in the exemplary processes of FIGS. 2B and 2C, the LiNixMn_(y)M_(z)Co_(1−x−y−z)O₂ is washed to provide a washed product, which is then heated in an oxygen-containing atmosphere at a temperature T_(S5) for an effective period of time t₅ to restore any lost oxygen from the crystal lattices (212 b,c, 213 b,c). The washing step (212 c) is optional in the streamlined process of FIG. 2C. In any of the foregoing or following embodiments, the temperature T_(S5) may be 500° C. to 800° C. and the effective period of time t₅ may be 1 hour to 25 hours. In some implementations, the temperature T_(S5) is 550° C. to 650° C. or 560° C. to 600° C. In certain examples, the temperature T_(S5) is 575° C. to 585° C., such as 580° C. In some implementations, the effective period of time t₅ is 1 to 15 hours, 1-10 hours, or 2 to 6 hours. In certain examples, the effective period of time t₅ is 3 hours to 5 hours, such as 4 hours. In any of the foregoing or following embodiments, the washed product may be dried before heating at the temperature T_(S5) for the effective period of time t₅. For example, the washed product may be dried at 60° C. to 120° C., 70° C. to 120° C., 60° C. to 95° C., or 70° C. to 90° C. under vacuum for 1 to 5 hours, such as at 80° C. for 2 hours.

In any of the foregoing or following embodiments, the solid hydroxide precursor may be prepared as discussed above. In any of the foregoing or following embodiments, the solid hydroxide precursor may have a mean particle size of 0.5 μm to 10 μm. In some embodiments, the hydroxide precursor particles have a mean size of 0.5 μm to 7.5 μm, 0.5 μm to 5 μm, or 0.5 μm to 2.5 μm. In any of the foregoing or following embodiments, monocrystalline lithium nickel manganese cobalt oxide may have a mean particle size of 0.5 μm to 5 μm. In some embodiments, the solid hydroxide precursor has a mean particle size of 1 μm to 2 μm. In certain embodiments, the monocrystalline lithium nickel manganese cobalt oxide has a mean particle size of 1 μm to 5 μm, or 1 μm to 3 μm. In any of the foregoing or following embodiments, the dopant metal(s) M may comprise Mg, Ti, Al, Zn, Fe, Zr, Sn, Sc, V, Cr, Fe, Cu, Zu, Ga, Y, Zr, Nb, Mo, Ru, Ta, W, Ir, or any combination thereof.

In some embodiments, LiNixMn_(y)M_(z)Co_(1−x−y−z)O₂ prepared from Li₂O and transition metal oxide precursors exhibits superior qualities compared to LiNixMn_(y)M_(z)Co_(1−x−y−z)O₂ prepared from other salts and/or by methods other than the streamlined method exemplified in FIG. 2C. For example, LiNixMn_(y)M_(z)Co_(1−x−y−z)O₂ prepared from Li₂O transition metal oxide precursors exhibits better slurry properties. When making electrodes, the LiNixMn_(y)M_(z)Co_(1−x−y−z)O₂ typically is mixed with a solvent, a binder, and optionally carbon to form a slurry, which is then coated onto the current collector. A common problem LiNixMn_(y)M_(z)Co_(1−x−y−z)O₂ slurries is gelation, making it difficult or even impossible to uniformly coat the slurry onto the current collector. Residual lithium compounds in the product absorb moisture and CO₂ from air, forming LiOH and Li₂CO₃. Without wishing to be bound by a particular theory of operation, the LiOH may trigger polyvinylidene fluoride (PVDF) binder gelation in the slurry by the following reaction (Cho et al., J. Elect. Chem. 2014, 161:A920-A926; Ross et al., Polymer2000, 41(5):1685-1696):

(CH₂—CF₂)_(n)+LiON→(CH═CF)_(n)+LiF+H₂O.

Even when washed and annealed, the LiNixMn_(y)M_(z)Co_(1−x−y−)zO₂ may still retain sufficient lithium salt impurities to absorb moisture and/or carbon dioxide upon standing, thereby inducing gelation in a slurry formed with the LiNixMn_(y)M_(z)Co_(1−x−y−z)O₂. The inventors surprisingly discovered that LiNixMn_(y)M_(z)Co_(1−x−y−z)O₂ prepared from Li₂O and transition metal oxide precursors eliminates the problem of slurry gelation. The slurry remains in a free flowing state and can be readily coated onto a current collector to form a uniform electrode. Without wishing to be bound by a particular theory of operation, the LiNixMn_(y)M_(z)Co_(1−x−y−z)O₂ prepared from Li₂O comprises little or no residual Li salts capable of absorbing moisture and/or carbon dioxide, thereby reducing or eliminating formation of LiOH and Li₂CO₃. In one embodiment, a slurry comprising the lithium nickel manganese cobalt oxide, a solvent, and a binder does not undergo gelation. In an independent embodiment, the lithium nickel manganese cobalt oxide is devoid, or substantially devoid, of lithium salts capable of reacting with water, carbon dioxide, or water and carbon dioxide. By “substantially devoid” is meant that the lithium nickel manganese cobalt oxide does not comprise residual Li salts in an amount sufficient to induce gelation of a slurry prepared from the lithium nickel manganese cobalt oxide, a solvent, and a binder.

C. Molten-Salt Method

Monocrystalline lithium nickel manganese cobalt oxide (or a doped variant thereof) may be synthesized by a molten-salt method. With reference to FIG. 3, in some embodiments, the molten-salt method comprises heating a solid hydroxide precursor comprising NixMn_(y)M_(z)Co_(1−x−y−z)(OH)₂ at a temperature T_(M1) in an oxygen-containing atmosphere for an effective period of time t₁ to convert the solid hydroxide precursor to a solid oxide precursor (301); combining the solid oxide precursor with a molar excess of a lithium compound and a sintering agent to form a mixture (302); heating the mixture in an oxygen-containing atmosphere at a temperature T_(M2) for a period of time t₂ (303); increasing the temperature to a temperature T_(M3), where T_(M3)>T_(M2), and heating the mixture at the temperature T_(M3) for a period of time t₃ to produce a first product and the sintering agent (304); cooling the first product and the sintering agent to ambient temperature (305); separating the sintering agent from the first product (306); drying the first product (307); and heating the first product in an oxygen-containing atmosphere at a temperature T_(M4) for an effective period of time t₄ to restore any lost oxygen in the lattices and produce monocrystalline lithium nickel manganese cobalt oxide having a formula LiNixMn_(y)M_(z)Co_(1−x−y−z)O₂ (308). In the foregoing formulas, M represents one or more dopant metals, x≥0.6, 0.01≤y<0.2, z≤0.05, and x+y+z≤1.0. More particularly, 0.62≤x+y+z≤1.0. In some embodiments, x=0.65-0.99, y=0.01-0.2, z=0-0.02, and x+y+z=0.66-1.0. In an independent embodiment, x=0.65-0.95, y=0.01-0.2, z=0-0.02, and x+y+z=0.7-0.98. In another independent embodiment, x=0.65-0.9, y=0.05-0.2, z=0-0.02, and x+y+z=0.7-0.95. In some examples, x is 0.7-0.9, such as 0.75-0.9 or 0.8-0.9; y is 0.05-0.15, such as 0.05-0.14 or 0.05-0.1; z is 0-0.02; and x+y+z is 0.8-0.98, such as 0.8-0.95.

In any of the foregoing or following embodiments, the temperature T_(M1) may be 400° C. to 1000° C. and/or the effective period of time t₁ may be 1 hour to 30 hours. In some embodiments, the temperature T_(M1) is 500° C. to 1000° C., 600° C. to 1000° C., 800° C. to 1000° C., or 850° C. to 950° C. In certain examples, the temperature T_(M1) was 800° C., 900° C., or 1000° C. Advantageously, the temperature T_(M1) is below a melting point of the hydroxide precursor. In any of the foregoing or following embodiments, the temperature may be increased to the temperature T_(M1) at a ramping rate of 1° C./minute to 300° C./minute, such as a ramping rate of 1° C./minute to 200° C./minute, 1° C./minute 100° C./minute, 1° C./minute 50° C./minute, 1° C./minute 25° C./minute, or 1° C./minute 15° C./minute. In one example, the ramping rate was 5° C./minute. The temperature T_(M1) is then maintained for the effective period of time t₁. In some embodiments, the effective period of time t₁ is 5 hours to 25 hours, 10 hours to 20 hours, or 12 hours to 18 hours. In certain examples, the effective period of time t₁ was 15 hours. In any of the foregoing or following embodiments, the oxygen-containing atmosphere may be air or pure oxygen. In some embodiments, the oxygen-containing atmosphere is air. In any of the foregoing or following embodiments, at a majority or all of the solid hydroxide precursor may be converted to the solid oxide precursor. In some embodiments, 90 wt % to 100 wt %, such as 95 wt % to 100 wt %, 97 wt % to 100 wt %, 98 wt % to 100 wt %, or 99 wt % to 100 wt % of the solid hydroxide precursor is converted to the solid oxide precursor. In certain embodiments, all of the solid hydroxide precursor is converted to the solid oxide precursor.

The solid oxide precursor is combined with a molar excess of a Li compound and a sintering agent to form a mixture. In any of the foregoing or following embodiments, the Li compound may comprise lithium hydroxide, lithium carbonate, lithium nitrate, lithium oxide, lithium peroxide, or any combination thereof. In any of the foregoing or following embodiments, the Li compound may have a mean particle size of 10 μm to 100 μm. In some embodiments, the Li compound comprises lithium oxide (Li₂O). In any of the foregoing or following embodiments, the solid oxide precursor and the lithium compound may be combined in a Li:solid oxide precursor molar ratio of 1:1 to 5:1, such as a molar ratio of 1:1 to 4:1, 1:1 to 3:1, 1:1 to 2:1, 1:1 to 1.5:1, 1.1:1 to 1.4:1, or 1.1:1 to 1.2:1. In any of the foregoing or following embodiments, the sintering agent may be NaCl or KCl. In some embodiments, the sintering agent is NaCl. NaCl may reduce the sintering temperature and/or time. In any of the foregoing or following embodiments, a weight ratio of the sintering agent to the combined solid oxide precursor and lithium compound may be 0.2:1 to 1:0.2, such as weight ratio of 0.3:1 to 1:0.3, 0.4:1 to 1:0.4, 0.5:1 to 1:0.5, 0.6:1 to 1:0.6, 0.7:1 to 1:0.7, 0.8:1 to 1:0.8, 0.85:1 to 1:0.85, or 0.9:1 to 1:0.9. In certain examples, the weight ratio was 1:1.

The mixture is heated in an oxygen-containing atmosphere at a temperature T_(M2) for a period of time t₂. In any of the foregoing or following embodiments, the temperature may be increased to the temperature T_(M2) at a ramping rate of 1° C./minute to 300° C./minute, such as a ramping rate of 1° C./minute to 200° C./minute, 1° C./minute to 100° C./minute, 1° C./minute to 50° C./minute, 5° C./minute to 25° C./minute, or 5° C./minute to 15° C./minute. In one example, the ramping rate was 10° C./minute. The temperature T_(M2) is then maintained for the effective period of time t₂. In any of the foregoing or following embodiments, the temperature T_(M2) may be 400-1000° C. and/or the effective period of time t₂ may be 1-30 hours. In some embodiments, the temperature T_(M2) is 500° C. to 1000° C., 600° C. to 1000° C., 700° C. to 900° C., or 750° C. to 850° C. In certain examples, the temperature T_(M2) was 800° C. In some embodiments, the effective period of time t₂ is 1 hour to 25 hours, 5 hours to 20 hours, or 5 hours to 15 hours. In certain examples, the effective period of time t₂ was 10 hours. In any of the foregoing or following embodiments, the oxygen-containing atmosphere may be air or pure oxygen. In some embodiments, the oxygen-containing atmosphere is pure oxygen.

The mixture then is heated at a temperature T_(M3) for a period of time t₃ to produce a first product and the sintering agent. The temperature T_(M3) is greater than the temperature T_(M2). In any of the foregoing or following embodiments, the temperature T_(M3) may be 600° C. to 1000° C. and/or the effective period of time t₃ may be 1-30 hours. In some embodiments, the temperature T_(M3) is 700° C. to 1000° C., 800° C. to 1000° C., or 850° C. to 950° C. In certain examples, the temperature T_(M3) was 900° C. In some embodiments, the effective period of time t₃ is 1 hour to 25 hours, 1 hour to 20 hours, 1 hour to 10 hours, or 2 hours to 6 hours. In certain examples, the effective period of time t₃ was 5 hours.

The first product and sintering agent are cooled to ambient temperature. In any of the foregoing or following embodiments, cooling the first product to ambient temperature may comprise cooling the first product to 20° C. to 30° C., such as to 20° C. to 25° C. In some embodiments, a mean particle size of the first product is reduced to 0.1 μm to 10 μm. In any of the foregoing or following embodiments, reducing a mean particle size of the first product to 0.1 μm to 10 μm may comprise grinding the first product to achieve the desired particle size.

The sintering agent is separated from the first product. In any of the foregoing or following embodiments, separating the sintering agent may comprise washing the sintering agent and the first product with a solvent in which the sintering agent is soluble and the first product is insoluble or substantially insoluble (e.g., less than 5 wt % of the first product is soluble in the solvent). In some embodiments, the solvent is water. In certain embodiments, washing the sintering agent and first product comprises stirring the ground sintering agent and first product in water and/or ultrasonicating the ground sintering agent and first product in the water. The resulting solution may be filtered to collect the first product. The first product then is dried to remove water. In any of the foregoing or following embodiments, drying the first product may comprise heating the first product at a temperature effective to evaporate the solvent for a time effective to remove the solvent, e.g., to remove at least 80 wt %, at least 90%, at least 95 wt %, at least 97 wt %, or at least 99 wt % of the solvent. In some embodiments, the solvent is water and the temperature is 60° C. to 95° C., such as 70° C. to 90° C. In certain embodiments, the first product may be heated under a reduced pressure to facilitate solvent removal. In any of the foregoing or following embodiments, the time may be from 1-10 hours, such as from 1-5 hours or from 1-3 hours. In some examples, the first product is heated under vacuum at 80° C. for 2 hours.

The first product is heated in an oxygen-containing atmosphere at a temperature T_(M4) for an effective period of time t₄ and produce monocrystalline lithium nickel manganese cobalt oxide having a formula LiNixMn_(y)M_(z)Co_(1−x−y−z)O₂. In any of the foregoing or following embodiments, the temperature T_(M4) may be 500° C. to 1000° C. and/or the effective period of time t₄ may be 1 hour to 30 hours. In some embodiments, the temperature T_(M4) is 500° C. to 800° C., 500° C. to 700° C., or 550° C. to 650° C. In certain examples, the temperature T_(M4) was 580° C. In some embodiments, the effective period of time t₄ is 1 hour to 25 hours, 1 hour to 20 hours, 1 hour to 10 hours, or 2 hours to 6 hours. In certain examples, the effective period of time t₄ was 4 hours. In any of the foregoing or following embodiments, the oxygen-containing atmosphere may be air or pure oxygen. In some embodiments, the oxygen-containing atmosphere is pure oxygen.

In any of the foregoing or following embodiments, the hydroxide precursor may be prepared as discussed above. In any of the foregoing or following embodiments, the solid hydroxide precursor may have a mean particle size of 0.5 μm to 10 μm. In some embodiments, the hydroxide precursor particles have a mean size of 0.5 μm to 7.5 μm, 0.5 μm to 5 μm, or 0.5 μm to 2.5 μm. In any of the foregoing or following embodiments, monocrystalline lithium nickel manganese cobalt oxide may have a mean particle size of 0.5 μm to 5 μm. In some embodiments, the solid hydroxide precursor has a mean particle size of 1 μm to 2 μm. In certain embodiments, the monocrystalline lithium nickel manganese cobalt oxide has a mean particle size of 1 μm to 5 μm, or 1 μm to 3 μm. In any of the foregoing or following embodiments, the dopant metal(s) M may comprise Mg, Ti, Al, Zn, Fe, Zr, Sn, Sc, V, Cr, Fe, Cu, Zu, Ga, Y, Zr, Nb, Mo, Ru, Ta, W, Ir, or any combination thereof.

D. Flash-Sintering Method

Monocrystalline lithium nickel manganese cobalt oxide (or a doped variant thereof) may be synthesized by a flash-sintering method. With reference to FIG. 4, in some embodiments, the flash-sintering method comprises combining a solid hydroxide precursor comprising NixMn_(y)M_(z)Co_(1−x−y−z)(OH)₂ with a molar excess of a lithium compound to form a hydroxide mixture (401); heating the hydroxide mixture in an oxygen-containing atmosphere at a temperature T_(F1) for an effective period of time t₁ to form an oxide mixture comprising oxides of nickel, manganese, cobalt, lithium, and, if present, the one or more dopant metals, or a combination thereof (402); increasing the temperature to a temperature T_(F2) at a rate of 10° C./min (403); and heating the oxide mixture in an oxygen-containing atmosphere at the temperature T_(F2) for an effective period of time t₂ to form monocrystalline lithium nickel manganese cobalt oxide having a formula LiNixMn_(y)M_(z)Co_(1−x−y−z)O₂ (404). In the foregoing formulas, M represents one or more dopant metals, x≥0.6, 0.01≤y<0.2, z≤0.05, and x+y+z≤1.0. More particularly, 0.62≤x+y+z≤1.0. In some embodiments, x=0.65-0.99, y=0.01-0.2, z=0-0.02, and x+y+z=0.7-1.0. In an independent embodiment, x=0.65-0.95, y=0.01-0.2, z=0-0.02, and x+y+z=0.7-0.98. In another independent embodiment, x=0.65-0.9, y=0.05-0.2, z=0-0.02, and x+y+z=0.7-0.95. In some examples, x is 0.7-0.9, such as 0.75-0.9 or 0.8-0.9; y is 0.05-0.15, such as 0.05-0.14 or 0.05-0.1; z is 0-0.02; and x+y+z is 0.8-0.98, such as 0.8-0.95.

The solid hydroxide precursor is combined with a molar excess of a Li compound to form a hydroxide mixture. In any of the foregoing or following embodiments, the Li compound may comprise lithium hydroxide, lithium carbonate, lithium nitrate, lithium oxide, lithium peroxide, or any combination thereof. In any of the foregoing or following embodiments, the Li compound may have a mean particle size of 10 μm to 100 μm. In some embodiments, the Li compound comprises lithium hydroxide. The LiOH may be anhydrous or a hydrated salt, e.g., LiOH.H₂O. In any of the foregoing or following embodiments, the solid hydroxide precursor and the lithium compound may be combined in a Li:solid hydroxide precursor molar ratio of 0.8:1 to 3:1, such as a molar ratio of 0.9:1 to 3:1, 0.9:1 to 2:1, 0.9:1 to 1.5:1, 1:1 to 1.5:1, 1.1:1 to 1.4:1, or 1.1:1 to 1.2:1.

The hydroxide mixture is heated in an oxygen-containing atmosphere at a temperature T_(F1) for an effective period of time t₁ to form an oxide mixture comprising oxides of nickel, manganese, cobalt, lithium, and, if present, the one or more dopant metals, or a combination thereof. In some embodiments, the hydroxide mixture is heated at the temperature T_(F1) in an absence of a sintering agent. In any of the foregoing or following embodiments, the temperature T_(F1) may be 400° C. to 1000° C. and/or the effective period of time t₁ may be 1 hour to 30 hours. In some embodiments, the temperature T_(F1) is 400° C. to 900° C., 400° C. to 800° C., 400° C. to 600° C., or 450° C. to 550° C. In certain examples, the temperature T_(F1) was 480° C. In any of the foregoing or following embodiments, the effective period of time t₁ is 1 hour to 30 hours. In some embodiments, the effective period of time t₁ is 1 hour to 25 hours, 1 hour to 20 hours, 1 hour to 15 hours, or 1 hour to 10 hours. In certain examples, the period of time t₁ was 5 hours. In any of the foregoing or following embodiments, the oxygen-containing atmosphere may be air or pure oxygen. In some embodiments, the oxygen-containing atmosphere is pure oxygen. In any of the foregoing or following embodiments, a portion or all of the hydroxide mixture is converted to an oxide mixture. In some embodiments, at least 25 wt %, at least 50 wt %, at least 75 wt %, or at least 90 wt % of the hydroxide mixture is converted to an oxide mixture. In certain embodiments, 25-100 wt %, 50-100 wt %, 75-100 wt %, 90-100 wt %, or 95-100 wt % of the hydroxide mixture is converted to an oxide mixture.

The temperature is then increased to a temperature T_(F2) at a rate of 0° C./minute. In any of the foregoing or following embodiments, the ramping rate may be from 10° C./minute to 3000° C./minute (50° C./second). In some embodiments, the ramping rate is from 10° C./minute to 2000° C./minute, such as 10° C./minute to 1000° C./minute, 10° C./minute to 500° C./minute, 10° C./minute to 250° C./minute, or 10° C./minute to 100° C./minute. In certain examples, the ramping rate was 10-20° C./minute. In any of the foregoing or following embodiments, the temperature T_(F2) may be 600° C. to 1000° C. In some embodiments, the temperature T_(F2) is 700° C. to 800° C. or 750° C. to 850° C. In some examples, the temperature T_(F2) was 800° C.

The oxide mixture is heated in an oxygen-containing atmosphere at the temperature T_(F2) for an effective period of time t₂ to form monocrystalline lithium nickel manganese cobalt oxide having a formula LiNixMn_(y)M_(z)Co_(1−x−y−z)O₂. In any of the foregoing or following embodiments, the effective period of time t₂ may be 1 hour to 30 hours. In some embodiments, the effective period of time t₂ is 1 hour to 25 hours, 1 hour to 20 hours, 5 hours to 20 hours, or 5 hours to 15 hours. In certain examples, the period of time t₂ was 10 hours. In any of the foregoing or following embodiments, the oxygen-containing atmosphere comprises pure oxygen or air. In some embodiments, the oxygen-containing atmosphere is pure oxygen.

In any of the foregoing or following embodiments, the hydroxide precursor may be prepared as discussed above. In any of the foregoing or following embodiments, the solid hydroxide precursor may have a mean particle size of 0.5 μm to 10 μm. In some embodiments, the hydroxide precursor particles have a mean size of 0.5 μm to 7.5 μm, 0.5 μm to 5 μm, or 0.5 μm to 2.5 μm. In any of the foregoing or following embodiments, monocrystalline lithium nickel manganese cobalt oxide may have a mean particle size of 0.5 μm to 5 μm. In some embodiments, the solid hydroxide precursor has a mean particle size of 1 μm to 2 μm. In certain embodiments, the monocrystalline lithium nickel manganese cobalt oxide has a mean particle size of 1 μm to 5 μm, or 1 μm to 3 μm. In any of the foregoing or following embodiments, the dopant metal(s) M may comprise Mg, Ti, Al, Zn, Fe, Zr, Sn, Sc, V, Cr, Fe, Cu, Zu, Ga, Y, Zr, Nb, Mo, Ru, Ta, W, Ir, or any combination thereof.

III. Cathodes and Lithium Ion Batteries

Monocrystalline lithium nickel manganese cobalt oxide (NMC), and doped variants thereof, made by embodiments of the disclosed methods may be used in cathodes, such as cathodes for lithium ion batteries. In some embodiments, a cathode comprises monocrystalline LiNixMn_(y)M_(z)Co_(1−x−y−z)O₂ where M represents one or more dopant metals, x≥0.6, 0.01≤y<0.2, z≤0.05, and x+y+z≤1.0. In some embodiments, x=0.65-0.99, y=0.01-0.2, z=0-0.02, and x+y+z=0.66-1.0. In an independent embodiment, x=0.65-0.95, y=0.01-0.2, z=0-0.02, and x+y+z=0.66-0.98. In another independent embodiment, x=0.65-0.9, y=0.05-0.2, z=0-0.02, and x+y+z=0.7-0.95. In some examples, x is 0.7-0.9, such as 0.75-0.9 or 0.8-0.9; y is 0.05-0.15, such as 0.05-0.14 or 0.05-0.1; z is 0-0.02; and x+y+z is 0.8-0.98, such as 0.8-0.95.

In any of the foregoing or following embodiments, a mean particle size of the monocrystalline LiNixMn_(y)M_(z)Co_(1−x−y−z)O₂ may be 0.5 μm to 5 μm, such as 1 μm to 5 μm, or 1 μm to 3 μm. In one embodiment, the NMC is LiNi_(0.76)Mn_(0.14)Co_(0.1)O₂. In an independent embodiment, the NMC is LiNi_(0.8)Mn_(0.1)Co_(0.1)O₂. In another independent embodiment, the NMC is LiNi_(0.9)Mn_(0.05)Co_(0.05)O₂. In still another independent embodiment, the NMC is LiNi_(0.76)Mn_(0.12)Co_(0.1) Mg_(0.01)Ti_(0.01)O₂.

In any of the foregoing or following embodiments, the cathode may have a capacity >180 mAh/g, In some embodiments, the cathode has a capacity >185 mAh/g, >190 mAh/g, or even >200 mAh/g. In any of the foregoing or following embodiments, the cathode may be operable at high voltage, e.g., a voltage of >3.8 V. In some embodiments, the cathode is operable at a voltage from 2-4.6 V, such as a voltage of 2-4.5 V or 2-4.4 V. In any of the foregoing or following embodiments, the cathode may have an NMC loading of 15-25 mg/cm², such as 18-24 mg/cm² (ca. 3.5-4.5 mAh/cm²). In some embodiments, the coating weight on each side of the cathode may be from 7.5-12.5 mg/cm², such as 9-12 mg/cm², providing an areal capacity on each side of 3.5-4.5 mAh/cm².

In any of the foregoing or following embodiments, the cathode may further comprise one or more inactive materials, such as binders and/or additives (e.g., carbon). In some embodiments, the cathode may comprise from 0-10 wt %, such as 2-5 wt % inactive materials. Suitable binders include, but are not limited to, polyvinyl alcohol, polyvinyl fluoride, ethylene oxide polymers, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, epoxy resin, nylon, polyimide and the like. Suitable conductive additives include, but are not limited to, carbon black, acetylene black, Ketjen black, carbon fibers (e.g., vapor-grown carbon fiber), metal powders or fibers (e.g., Cu, Ni, Al), and conductive polymers (e.g., polyphenylene derivatives). In some embodiments, a slurry comprising the NMC and, optionally, inactive materials is coated onto a support, such as aluminum foil. In certain embodiments, the coating may have a thickness of 50-80 μm on each side, such as a thickness of 60-70 μm. In any of the foregoing or following embodiments, the cathode may have an electrode press density of 2.5-3.5 g/cm³, such as 3 g/cm³.

In some embodiments, a lithium ion battery includes a cathode comprising monocrystalline NMC as disclosed herein, an anode, an electrolyte, and optionally a separator. FIG. 5 is a schematic diagram of one exemplary embodiment of a rechargeable battery 500 including a cathode 520 as disclosed herein, a separator 530 which is infused with an electrolyte, and an anode 540. In some embodiments, the battery 500 also includes a cathode current collector 510 and/or an anode current collector 550. The electrolyte may be any electrolyte that is compatible with the anode and suitable for use in a lithium ion battery.

In any of the foregoing or following embodiments, the lithium ion battery may be a pouch cell. FIG. 6 is a schematic side elevation view of one embodiment of a simplified pouch cell 600. The pouch cell 600 comprises an anode 610 comprising graphite material, an anode current collector 630, a cathode 640 comprising an NMC cathode material 650 as disclosed herein and a cathode current collector 660, a separator 670, and a packaging material defining a pouch 680 enclosing the anode 610, cathode 640, and separator 670. The pouch 680 further encloses an electrolyte (not shown). The anode current collector 630 has a protruding tab 631 that extends external to the pouch 680, and the cathode current collector 660 has a protruding tab 661 that extends external to the pouch 680. The pouch cell weight includes all components of the cell, i.e., anode, cathode, separator, electrolyte, and pouch material. In some embodiments, the pouch cell has a ratio of anode (negative electrode) areal capacity to cathode (positive electrode) areal capacity—N/P ratio—of 0.02-5, 0.1-5, 0.5-5, or 1-5. In certain embodiments, the pouch cell has a ratio of electrolyte mass to cell capacity—E/C ratio—of 1-6 g/Ah, such as 2-6 g/Ah or 2-4 g/Ah.

In any of the foregoing or following embodiments, the current collectors can be a metal or another conductive material such as, but not limited to, nickel (Ni), copper (Cu), aluminum (Al), iron (Fe), stainless steel, or conductive carbon materials. The current collector may be a foil, a foam, or a polymer substrate coated with a conductive material. Advantageously, the current collector is stable (i.e., does not corrode or react) when in contact with the anode or cathode and the electrolyte in an operating voltage window of the battery. The anode and cathode current collectors may be omitted if the anode or cathode, respectively, are free standing, e.g., when the anode is a free-standing film, and/or when the cathode is a free-standing film. By “free-standing” is meant that the film itself has sufficient structural integrity that the film can be positioned in the battery without a support material.

In any of the foregoing or following embodiments, the anode may be any anode suitable for a lithium ion battery. In some embodiments, the anode is lithium metal, graphite, an intercalation material, or a conversion compound. The intercalation material or conversion compound may be deposited onto a substrate (e.g., a current collector) or provided as a free-standing film, typically, including one or more binders and/or conductive additives. Suitable binders include, but are not limited to, polyvinyl alcohol, polyvinyl chloride, polyvinyl fluoride, ethylene oxide polymers, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, epoxy resin, nylon, polyimide and the like. Suitable conductive additives include, but are not limited to, carbon black, acetylene black, Ketjen black, carbon fibers (e.g., vapor-grown carbon fiber), metal powders or fibers (e.g., Cu, Ni, Al), and conductive polymers (e.g., polyphenylene derivatives). Exemplary anodes for lithium batteries include, but are not limited to, lithium metal, carbon-based anodes (e.g., graphite, silicon-based anodes (e.g., porous silicon, carbon-coated porous silicon, carbon/silicon carbide-coated porous silicon), Mo₆S₈, TiO₂, V₂O₅, Li₄Mn₅O₁₂, Li₄Ti₅O₁₂, C/S composites, and polyacrylonitrile (PAN)-sulfur composites. In some embodiments, the anode is lithium metal. In certain embodiments, the anode may have a lithium coating weight on each side of a current collector of 5-15 mg/cm², providing an areal capacity on each side of 2-5.1 mAh/cm².

In any of the foregoing or following embodiments, the separator may be glass fiber, a porous polymer film (e.g., polyethylene- or polypropylene-based material) with or without a ceramic coating, or a composite (e.g., a porous film of inorganic particles and a binder). One exemplary polymeric separator is a Celgard® K1640 polyethylene (PE) membrane. Another exemplary polymeric separator is a Celgard® 2500 polypropylene membrane. Another exemplary polymeric separator is a Celgard® 3501 surfactant-coated polypropylene membrane. The separator may be infused with the electrolyte.

In any of the foregoing or following embodiments, the electrolyte may comprise a lithium active salt and a solvent. In some embodiments, the lithium active salt comprises LiPF₆, LiAsF₆, LiBF₄, lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(oxalato)borate (LiB(C₂O₄)₂, LiBOB), lithium difluoro(oxalato)borate (LiBF₂(C₂O₄), LiDFOB), lithium bis(pentafluoroethanesulfonyl)imide (LiN(SO₂CF₂CF₃)₂, LiBETI), lithium (fluorosulfonyl trifluoromethanesulfonyl)imide (LiN(SO₂F)(SO₂CF₃), LiFTFSI), lithium (fluorosulfonyl pentafluoroethanesulfonyl)imide (LiN(SO₂F)N(SO₂CF₂CF₃), LiFBETI), lithium cyclo(tetrafluoroethylenedisulfonyl)imide (LiN(SO₂CF₂CF₂SO₂), LiCTFSI), lithium (trifluoromethanesulfonyl)(n-nonafluorobutanesulfonyl)imide (LiN(SO₂CF₃)(SO₂-n-C₄F₉), LiTNFSI), lithium cyclo-hexafluoropropane-1,3-bis(sulfonyl)imide, or any combination thereof. The solvent is any nonaqueous solvent suitable for use with the lithium active salt, lithium metal anode, and packaging material. Exemplary solvents include, but are not limited to, triethyl phosphate, trimethyl phosphate, tributyl phosphate, triphenyl phosphate, tris(2,2,2-trifluoroethyl) phosphate, bis(2,2,2-trifluoroethyl) methyl phosphate; trimethyl phosphite, triphenyl phosphite, tris(2,2,2-trifluoroethyl) phosphite; dimethyl methylphosphonate, diethyl ethylphosphonate, diethyl phenylphosphonate, bis(2,2,2-trifluoroethyl) methylphosphonate; hexamethylphosphoramide; hexamethoxyphosphazene, hexafluorophosphazene, 1,2-dimethoxyethane (DME), 1,3-dioxolane (DOL), tetrahydrofuran (THF), allyl ether, dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), ethylene carbonate (EC), propylene carbonate (PC), vinylene carbonate (VC), fluoroethylene carbonate (FEC), 4-vinyl-1,3-dioxolan-2-one (vinyl ethylene carbonate, VEC), 4-methylene-1,3-dioxolan-2-one (methylene ethylene carbonate, MEC), 4,5-dimethylene-1,3-dioxolan-2-one, dimethyl sulfoxide (DMSO), dimethyl sulfone (DMS), ethyl methyl sulfone (EMS), ethyl vinyl sulfone (EVS), tetramethylene sulfone (i.e. sulfolane, TMS), trifluoromethyl ethyl sulfone (FMES), trifluoromethyl isopropyl sulfone (FMIS), trifluoropropyl methyl sulfone (FPMS), diethylene glycol dimethyl ether (diglyme), triethylene glycol dimethyl ether (triglyme), tetraethylene glycol dimethyl ether (tetraglyme), methyl butyrate, ethyl propionate, gamma-butyrolactone, acetonitrile (AN), succinonitrile (SN), adiponitrile, triallyl amine, triallyl cyanurate, triallyl isocyanurate, or any combination thereof. In some embodiments, the solvent comprises a flame retardant compound. The flame retardant compound may comprise the entire solvent. Alternatively, the solvent may comprise at least 5 wt % of the flame retardant compound in combination with one or more additional solvents and/or diluents. Exemplary flame retardant compounds include, but are not limited to, triethyl phosphate, trimethyl phosphate, tributyl phosphate, triphenyl phosphate, tris(2,2,2-trifluoroethyl) phosphate, bis(2,2,2-trifluoroethyl) methyl phosphate; trimethyl phosphite, triphenyl phosphite, tris(2,2,2-trifluoroethyl) phosphite; dimethyl methylphosphonate, diethyl ethylphosphonate, diethyl phenylphosphonate, bis(2,2,2-trifluoroethyl) methylphosphonate; hexamethylphosphoramide; hexamethoxyphosphazene, hexafluorophosphazene, and combinations thereof. In some embodiments, the electrolyte has a lithium active salt concentration of 0.5-8 M, such as a concentration of 1-8 M, 1-6 M, or 1-5 M. In some examples, the electrolyte comprises LiPF₆ in a carbonate solvent, such as 1.0 M LiPF₆ in EC/EMC.

In some embodiments, the electrolyte is a localized superconcentrated electrolyte (LSE), also referred to as a localized high concentration electrolyte. A LSE includes an active salt, a solvent in which the active salt is soluble, and a diluent, wherein the active salt has a solubility in the diluent at least 10 times less than a solubility of the active salt in the solvent. In an LSE, lithium ions remain associated with solvent molecules after addition of the diluent. The anions are also in proximity to, or associated with, the lithium ions. Thus, localized regions of solvent-cation-anion aggregates are formed. In contrast, the lithium ions and anions are not associated with the diluent molecules, which remain free in the solution. In an LSE, the electrolyte as a whole is not a concentrated electrolyte, but there are localized regions of high concentration where the lithium cations are associated with the solvent molecules. There are few to no free solvent molecules in the diluted electrolyte, thereby providing the benefits of a superconcentrated electrolyte without the associated disadvantages. The solubility of the active salt in the solvent (in the absence of diluent) may be greater than 3 M, such as at least 4 M or at least 5 M. In some embodiments, the solubility and/or concentration of the active salt in the solvent is of 3 M to 10 M, such as from 3 M to 8 M, from 4 M to 8 M, or from 5 M to 8 M. However, in some embodiments, the molar concentration of the active salt in the LSE as a whole is of 0.5 M to 3 M, 0.5 M to 2 M, 0.75 M to 2 M, or 0.75 M to 1.5 M.

Exemplary salts and solvents for LSEs are those disclosed above. In some embodiments, the diluent comprises a fluoroalkyl ether (also referred to as a hydrofluoroether (HFE)), a fluorinated orthoformate, or a combination thereof. Exemplary diluents include, but are not limited to, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE), bis(2,2,2-trifluoroethyl) ether (BTFE), 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether (TFTFE), methoxynonafluorobutane (MOFB), ethoxynonafluorobutane (EOFB), tris(2,2,2-trifluoroethyl)orthoformate (TFEO), tris(hexafluoroisopropyl)orthoformate (THFiPO), tris(2,2-difluoroethyl)orthoformate (TDFEO), bis(2,2,2-trifluoroethyl) methyl orthoformate (BTFEMO), tris(2,2,3,3,3-pentafluoropropyl)orthoformate (TPFPO), tris(2,2,3,3-tetrafluoropropyl)orthoformate (TTPO), or any combination thereof. In certain embodiments where the diluent and solvent are immiscible, the electrolyte may further include a bridge solvent having a different composition than the solvent and a different composition than the diluent, wherein the bridge solvent is miscible with the solvent and with the diluent. Exemplary bridge solvents include acetonitrile, dimethyl carbonate, diethyl carbonate, propylene carbonate, dimethyl sulfoxide, 1,3-dioxolane, dimethoxyethane, diglyme (bis(2-methoxyethyl) ether), triglyme (triethylene glycol dimethyl ether), tetraglyme (tetraethylene glycol dimethyl ether), or any combination thereof. Additional information regarding LSEs may be found in US 2018/0254524 A1, US 2018/0251681 A1, and US 2019/148775 A1, each of which is incorporated in its entirety herein by reference.

In any of the foregoing or following embodiments, the lithium ion battery may have a cell energy density of 200 Wh/kg to 400 Wh/kg, such as 200 Wh/kg to 350 Wh/kg, 250 Wh/kg to 300 Wh/kg, or 250 Wh/kg to 275 Wh/kg. In any of the foregoing or following embodiments, the lithium ion battery may have a first cycle capacity of 2 Ah to 5 Ah. In any of the foregoing or following embodiments, the lithium ion battery may be operable at a rate of up to 3C, such as a rate of 0.1C to 3C. In any of the foregoing or following embodiments, the lithium ion battery may demonstrate an average coulombic efficiency of at least 80%, such as 80-100%, 85-100%, 90-100%, 95-100%, or 95-99% over at least 100 cycles, at least 150 cycles, or at least 200 cycles. In any of the foregoing or following embodiments, the lithium ion battery may have a capacity retention of at least 70%, at least 75%, or at least 80% after at least 100 cycles, at least 150 cycles, or at least 200 cycles. In some embodiments, the capacity retention is 70-90%, such as 70-85%, after 200 cycles. Advantageously, the cathode may be more stable (e.g., resistant to cracking) over extensive cycling, undergo fewer side reactions, be less moisture sensitive, and/or generate less gas during cycling compared to conventional Ni-rich cathodes (e.g., Ni≥0.6) prepared with polycrystalline and/or monocrystalline NMC.

IV. Representative Embodiments

Certain representative embodiments are exemplified in the following paragraphs.

A method comprising: making lithium nickel manganese cobalt oxide by combining a molar excess of Li₂O with a solid oxide precursor comprising NixMn_(y)M_(z)Co_(1−x−y−z)O₂, where M represents one or more dopant metals, x≥0.6, 0.01≤y<0.2, 0≤z≤0.05, and x+y+z≤1.0; heating the solid oxide precursor and the Li₂O in an oxygen atmosphere at a first temperature T₁ for a first effective period of time t₁ to produce a first product; washing the first product to remove any impurities and provide a washed product; and heating the washed product in an oxygen atmosphere at a second temperature T₂ for a second effective period of time t₂ to produce lithium nickel manganese cobalt oxide having a formula LiNixMn_(y)M_(z)Co_(1−x−y−z)O₂.

The method of the foregoing paragraph, wherein the Li₂O has a mean particle size of less than or equal to 150 μm.

The method of either of the foregoing paragraphs, wherein the dopant metal M, if present, comprises Mg, Ti, Al, Zn, Fe, Zr, Sn, Sc, V, Cr, Fe, Cu, Zu, Ga, Y, Zr, Nb, Mo, Ru, Ta, W, Ir, or any combination thereof.

The method of any of the foregoing paragraphs, wherein z is 0, and the lithium nickel manganese cobalt oxide has a formula LiNixMn_(y)Co_(1−x−y)O₂.

The method of any of the foregoing paragraphs, wherein: (i) the first temperature T₁ is 800° C. to 1000° C.; or (ii) the first effective period of time t₁ is 1 hour to 30 hours; or (iii) both (i) and (ii).

The method of any of the foregoing paragraphs, wherein: (i) the temperature T₁ is 900° C. to 1000° C.; (ii) the effective period of time t₁ is 5 hours to 15 hours; or (iii) both (i) and (ii).

The method of any of the foregoing paragraphs, wherein: (i) the temperature T₁ is 950° C. to 1000° C.; (ii) the effective period of time t₁ is 8 hours to 12 hours; or (iii) both (i) and (ii).

The method of any of the foregoing paragraphs, wherein: (i) the second temperature T₂ is 500° C. to 800° C.; or (ii) the second effective period of time t₂ is 1 hour to 25 hours; or (iii) both (i) and (ii).

The method of any of the foregoing paragraphs, wherein: (i) the temperature T₂ is 550° C. to 650° C.; (ii) the effective period of time t₂ is 2 hours to 6 hours; or (iii) both (i) and (ii). The method of any of the foregoing paragraphs, wherein: x=0.65-0.9; y=0.05-0.2; z=0-0.02; and x+y+z=0.7-0.95.

The method of any of the foregoing paragraphs, wherein: x=0.75-0.9; y=0.05-0.14; z=0-0.02; and x+y+z=0.8-0.98.

The method of any of the foregoing paragraphs, wherein the solid oxide precursor and the Li₂O are combined in a Li:solid oxide precursor molar ratio of 1.01:1 to 3:1.

The method of any of the foregoing paragraphs, further comprising heating the washed product at a temperature of 60° C. to 120° C. for a time of 1 hour to 10 hours prior to heating the washed product at the temperature T₂ for the effective period of time t₂.

The method of any of the foregoing paragraphs, wherein the first temperature T₁ is 925° C. to 1000° C., and the lithium nickel manganese cobalt oxide is monocrystalline.

The method of any of the foregoing paragraphs, wherein the oxygen atmosphere comprises pure oxygen or air.

The method of any of the foregoing paragraphs, further comprising preparing the solid oxide precursor by: preparing a 1.5-2.5 M solution comprising metal salts in water, the metal salts comprising a nickel (II) salt, a manganese (II) salt, a cobalt (II) salt, and optionally one or more dopant metal salts, wherein a mole fraction x of the nickel (II) salt in the solution is 0.6, a mole fraction y of the manganese (II) salt is 0.01≤y<0.2, a mole fraction z of the one or more dopant metal salts is 0≤z≤0.05, a mole fraction of the cobalt (II) salt is 1−x−y−z, and x+y+z≤1.0; combining the solution comprising metal salts in water with aqueous NH₃ and aqueous NaOH or KOH to provide a combined solution having a pH of 10.5-12 and a combined metal salt concentration of 0.1 M to 3 M; aging the combined solution for 5 hours to 48 hours at a temperature of 25° C. to 80° C. to co-precipitate hydroxides of nickel, manganese, and cobalt to provide a solid hydroxide precursor comprising NixMn_(y)M_(z)Co_(1−x−y−z)(OH)₂; heating the solid hydroxide precursor comprising NixMn_(y)M_(z)Co_(1−x−y−z)(OH)₂ at a temperature of 500° C. to 1000° C. in an oxygen-containing atmosphere for 1 hour to 30 hours to convert the solid hydroxide precursor to the solid oxide precursor.

A method comprising: making monocrystalline lithium nickel manganese cobalt oxide by combining a molar excess of Li₂O with a solid oxide precursor comprising NixMn_(y)M_(z)Co_(1−x−y−z)O₂, where M represents one or more dopant metals, x≥0.6, 0.01≤y<0.2, 0≤z≤0.05, and x+y+z≤1.0; heating the solid oxide precursor and the Li₂O in an oxygen atmosphere at 800° C. to 1000° C. for 5 hours to 15 hours to produce a first product; cooling the first product to ambient temperature; and heating the first product in an oxygen atmosphere at 500° C. to 800° C. for 2 hours to 6 hours to produce lithium nickel manganese cobalt oxide having a formula LiNixMn_(y)M_(z)Co_(1−x−y−z)O₂.

The method of the foregoing paragraph, wherein the solid oxide precursor and the Li₂O are heated in the oxygen atmosphere at 800° C. to 950° C. for 8 hours to 12 hours.

The method of either of the foregoing paragraphs, wherein z is 0, and the product is LiNixMn_(y)Co_(1−x−y)O₂.

The method of any of the foregoing paragraphs, wherein: x=0.65-0.9; y=0.05-0.2; z=0-0.02; and x+y+z=0.7-0.95.

V. Examples Example 1 Molten-Salt Synthesis and Characterization of Single Crystal LiNi_(0.76)M_(0.14)C_(0.1)O₂ (NMC76) Synthesis

A Ni_(0.76)Mn_(0.14)Co_(0.1)(OH)₂ precursor was synthesized by co-precipitation method using a 5 L reactor at 55° C. Before the co-precipitation reaction, 1.5 L deionized (DI) H₂O and 65 mL concentrated NH₃.H₂O (˜28 wt %) were added to the reactor and heated to 55° C. as a starting solution. A 2 mol/L transition metal (TM) sulfate solution (Ni:Mn:Co=0.76:0.14:0.10 in molar ratio) was prepared with Ni(SO₄)₂.6H₂O, MnSO₄.H₂O, and Co(SO₄)₂.7H₂O, and then pumped into reactor along with 8 M NaOH as well as 10 mol/L NH₃.H₂O buffer solution. The pumping rates of the TM sulfate and NH₃.H₂O solution were set at 3 and 1 mL/min, respectively. The pH value was controlled at 11.2 during reaction by adjusting NaOH adding rate. The synthesized precursor was filtered and washed by DI H₂O to remove impurities. After drying at 100° C. for 12 hours, the Ni_(0.76)Mn_(0.14)Co_(0.1)(OH)₂ precursor was obtained. Ni_(0.76)Mn_(0.14)Co_(0.1)(OH)₂ was heated at 900° C. for 15 hours in air to prepare the oxide precursor. The stoichiometry of the oxide precursor was analyzed by ICP-OES. The oxide precursor and lithium oxide were mixed at 1:0.6 molar ratio (TM:Li=1:1.2), then NaCl (1:1.1 weight ratio) was added to the precursor and mixed uniformly. The mixture was sintered at 800° C. for 10 hours and then 900° C. for 5 hours using a 10° C./minute ramping rate in an oxygen atmosphere. The sintered products were washed and dried at 80° C. in vacuum for 2 hours. Post annealing at 580° C. (10° C./min ramping) in oxygen atmosphere was carried out before collecting the end products. Pure NaCl was used as the molten salt media to lower the sintering time/temperature of the single crystals, thereby reducing the cost of synthesizing the single crystals.

Characterization

Scanning electron images (SEM) were collected on a Helios NanoLab scanning electron microscope (FEI, Hillsboro, Oreg.). Powder X-ray diffraction (XRD) data were collected at the 28-ID-2 (XPD) beamline of the National Synchrotron Light Source II (NSLS II), Brookhaven National Laboratory (BNL). A Perkin Elmer amorphous-Si flat panel detector was used. The wavelength was 0.1821 Å. Pristine powder was loaded in Kapton® polyimide capillary tubes (1.0 mm diameter) in an Ar-filled glove box and mounted on bases at the beamline. Rietveld refinement of the XRD data was carried out in TOPAS6 (Coelho, J. Appl. Crystallogr. 2018, 51:210-218). Transmission electron microscopy (TEM) specimen preparation was conducted on a FEI Helios Dual-Beam FIB operating at 2-30 kV. TEM images, selected area electron diffraction (SAED), high resolution TEM (HRTEM) images and scanning TEM energy-dispersive X-ray spectroscopy (STEM-EDS) were performed on a FEI Titan 80-300 S/TEM microscope at 300 kV which was equipped with a probe spherical aberration corrector. HRSTEM high-angle annular dark-field (HRSTEM-HAADF) images and STEM-electron energy loss spectroscopy (STEM-EELS) were performed on a probe aberration-corrected JEOL JEM-ARM200CF microscope (JEOL USA, Peabody, Mass.) at 200 kV.

Since the TEM sample was very thin and self-absorption in the TEM sample was negligible, EDS quantitation was performed by using a simple ratio technique (Cliff-Lorimer quantification). In this method, peak intensities are proportional to concentration and specimen thickness, and the effects of variable specimen thickness are removed by taking ratios of intensities for elemental peaks and introduced a “k-factor” to relate the intensity ratio to concentration ratio:

C _(A) /C _(B) =k _(AB) ·I _(A) /I _(B)  (1)

Where I_(A) is Peak intensity for element A and C_(A) is concentration in weight %. Each pair of elements requires a different k-factor which depends on detector efficiency, ionization cross section and fluorescence yield of the two elements concerned. k-factor kab is calculated as follows:

k _(ab)=(A _(a)ω_(b) Q _(b))/(A _(b)ω_(a) Q _(a))  (2)

Where A_(a) and A_(b) are the atomic weights of elements giving rise to the analytical lines a and b respectively, Q_(a) and Q_(b) are the ionization cross sections of the shells, that once ionized give rise to the analytical lines at the specified accelerating voltage. The fluorescent yields ω_(a) and ω_(b) give probability of emission of the lines once the appropriate shell has been ionized. Once the k factor is known, the element concentration in weight % can be calculated.

The as-prepared single crystalline NMC76 had a tap density of 2.12 mg/cm³, while polycrystalline NMC76 is 2.08 mg/cm³. The increased tap density will benefit the cell-level energy due to the reduced porosity and improved electrode press density.

Electrochemical Test

To understand the electrochemical properties of single crystal NMC76 at industry relevant scales, a cathode with reasonably high loading is needed. However, half cells are not a good testing vehicle to evaluate high mass loading of the cathode materials due to the accelerated Li metal degradation when coupled with a thick cathode. Electrochemical performance was evaluated with 2032 type coin cells. Single crystalline MNC76 was mixed with carbon additive (C65 carbon black) and PVDF at 96:2:2 weight ratio in NMP (N-methyl-2-pyriolidone) solvent and loaded on carbon coated Al foil. The areal loading of 5-26 mg/cm² electrodes were prepared by adjusting the height of doctor blade. After drying at 80° C. under vacuum, thick electrodes (˜20 mg/cm²) were calendared (˜32% porosity) and cut into CP ½ inch size discs for assembling cells. Prior to calendaring, the porosity was 62%. Graphite powder was mixed with carbon additive (C65), CMC and SBR at weight ratio of 94.5:1:2.25:2.25 and loaded on copper foil. The dried graphite electrodes were calendared and cut into ϕ 15 mm size discs. 1.0 M LiPF₆ in EC/EMC (3:7 weight ratio) with 2 wt % VC was used as electrolyte for both half cell and full cell tests. Half cells were assembling using 450 μm thick Li metal as anode. Negative/Positive (N/P) ratio for full cells assembling was controlled 1.15˜1.2 (including ˜0.1 mg Li metal in graphite anode). In all the cell tests, 1C is named as 200 mA/g.

In Situ Electrochemical AFM Measurements (EC-AFM)

Fabrication of working electrodes for in situ EC-AFM: Aluminum (Al) foil was chosen as the substrate because of its electronic conductivity and stability during the electrochemical cycles. The synthesized Ni-rich NMC single crystals were dropped on Al foil. Then the Al foil with Ni-rich NMC was placed under a 5 tonnage press machine and held for 100 s. The weakly bound and unfixed Ni-rich NMC crystals were removed by flowing N₂ gas. The Al foil was then mounted onto the AFM sample holder with epoxy and an electrical contact was made to the bottom side of Al foil with a flattened nickel wire. Silicone grease was used to cover the outer rim of the O-ring to enhance the sealing of solution. The nickel wire was isolated from solution due to the larger diameter of Al foil than O-ring. The cell was ready for in situ AFM testing after cell assembly with an O-ring, sealing cap with Li wire for the counter and reference electrode, and another free sealing cap.

All in situ EC-AFM images were captured in peak force or tapping mode at room temperature (23° C.) with a Nanoscope 8 atomic force microscope (J scanner, Bruker, Santa Barbara, Calif.) (Habraken et al., Nat Commun 2013, 4:1507; Tao et al., PNAS USA 2019, 116:13867-13872). The AFM probe consisted of silicon tips on silicon nitride cantilevers (HA_C Series of ETALON probes, k=0.26 N/m, tip radius <10 nm; K-TEK Nanotechnology, https://kteknano.com/product-category/etalon/). The electrolyte solution of 1.0 M LiPF₆ in EC/EMC (3:7 weight ratio) was used as the testing solution. A working cathode electrode (described above) was connected to Solartron 1287 electrochemistry workstation (Solartron, Farnborough, Hampshire, UK) by the nickel wire. A thin Li wire (counter and reference electrode) with sealing cap was inserted into the testing solution via another channel in liquid cell (Lv et al., Nano Lett 2017, 17:1602-1609). An electric signal was input into an AFM liquid cell with a two-electrode configuration by a Solartron 1287 electrochemistry workstation. The applied voltage started at OCV, increased up to 4.50 V and down to 2.70 V with a uniform scan rate of 0.3 mV/s. For typical imaging conditions, images were collected at scanning speeds of 1 Hz.

Several protocols were followed to ensure that the AFM images were typical representations of the surface topography evolution. First, the imaging force was reduced to the minimum possible value (˜100 pN) that still allowed the tip to track the surface and no measurable effect of the scanning on the surface cracks. We verified this by zooming out to a larger scan box and comparing the crack number density with the smaller scan area. A consequence of imaging in the same area is that it may cause the less bound particles to move on the surface. This operation ensures that the crack growth kinetics are minimally affected. Images were also collected at different scan angles and trace and retrace images regularly compared to eliminate the possibility of imaging artifacts from tip contamination. The images were analyzed using the image processing software package Nanoscope Analysis 2.0 (Bruker).

The average width of these steps almost linearly increases with the higher voltage (vs. Li+/Li) during charging process followed by linear decreases with the lower voltage during the discharge process, with its value of 30.3±0.7 nm at open circuit voltage (OCV), up to 52.5±2.5 nm at 4.50 V at charge status, and down to 38.4±1.3 nm at 4.19 V during discharge. The average step width is calculated by dividing the total width of the lateral face by the total number of steps (between 21 and 43 steps) for each voltage during the in situ AFM monitoring. The error bars in the step width are evaluated by averaging of three times of measurement of lateral face width at each time point.

Simulation

A cylindrical electrode particle diffusion-induced-stress model was used here along with material properties predicted by density functional theory (DFT) (28, 33). The particle is considered as an isotropic solid with Young's modulus (E) increasing linearly with Li concentration. In the layered Ni-rich electrode, the diffusion of lithium ion is limited in the two-dimensional channels, namely, between the layers, so lithium diffusion along the radial direction is assumed in the model. The dimensionless particle size, time, Li concentration, and stress are used following the definitions in Deshpande et al., J Electrochem Soc 2010, 157:A967-A971). For the parameters in simulation, α=0.0067; E₀=59.8 GPa; E=E₀+204.2C; v=0.3 (Qi et al. J Electrochem Soc 2014, 161:F3010-F3018); and γ=2.1 J/m² was computed for Li₁₆(Ni₁₄CoMn)O₃₂ (Stein et al., Acter Mater 2018, 159:225-240). The analytical solutions are provided in FIGS. 29A-29D, 30A-30D. The numerical solution by COMSOL5.5 model is provided in AGS. 31A-31D. For the anisotropic chemical strain solved in FIG. 31D, the α=(0, 0, 0.02).

Results and Discussion

The synthesized NMC76 has a mean particle size of 3 μm (FIG. 7A). A cross-section view (FIG. 17B) shows that NMC76 has a dense structure without cavities or grain boundaries. Pure phases of α-NaFeO₂-type layered structures are confirmed by both selected area electron diffraction (SAED, FIG. 7C) and X-ray Diffraction (XRD, FIG. 7D). Lattice parameters a and c are 2.8756(1) Å and 14.2221(1) Å, respectively, from Rietveld refinement (Table 1). For comparison, polycrystalline NMC76 is found to contain many internal pores and intergranular boundaries along with surface films (FIGS. 8A-8F) formed from the reactions between NMC and air (Jung et al., J Electrochem Soc 2018, 165:A132-A141). In contrast, the surface of single crystalline NMC76 is very uniform (FIGS. 7E and 7F). Elemental mapping (FIGS. 7G and 7H) indicates a homogeneous distribution of Ni, Mn and Co with a stoichiometric ratio as designed (Table 2). Continuous phase transitions happen when potential changes (FIG. 9), similar to polycrystalline NMC76 (Zheng et al., Nano Energy 2018, 49:538-548). During charge, phase transitions happen in the order of from H1 to M (H and M are hexagonal phase and monoclinic phase, respectively), M to H2, and H2 to H3, similar to polycrystalline NMC76.

TABLE 1 RietveId refinement result of the pristine NMC76. a = 2.8756(1) Å, c = 14.2221(1) Å Atom Site x y z Fraction Uiso Li 3a 0 0 0   0.986(1) 0.004(1) Ni 3a 0 0 0   0.014(1) 0.004(1) Ni 3b 0 0 0.5 0.746(1) 0.005(2) Li 3b 0 0 0.5 0.014(1) 0.005(2) Mn 3b 0 0 0.5 0.14 0.005(2) Co 3b 0 0 0.5 0.1  0.005(2) O 6c 0 0 0.2403(1) 1   0.009(4)

TABLE 2 Elemental Ratio obtained from STEM-EDS Element Wt % Stdev Ni 48.28 1.45 Mn 7.62 0.9 Co 6.98 1.13 O 37.12 1.69

Single crystalline NMC76 was further tested in graphite/NMC full cells at realistic conditions. The typical loading of NMC76 cathodes is ca. 20 mg/cm² (=4 mAh/cm²) with ca. 32% porosity, which is needed to build a 250 Wh/kg Li-ion cell (Table 3). At such a high cathode loading, Li metal will worsen the cycling stability (FIGS. 10, 11) due to the deepened stripping/deposition process of Li. Between 2.7 V and 4.2 V (vs. graphite), single crystalline NMC76 delivered 182.3 mAh/g discharge capacity at 0.1C, and retained 86.5% of its original capacity after 200 cycles (FIG. 12A). With a cutoff of 4.3 V, single crystalline NMC76 delivered 193.4 mAh/g capacity with 81.6% capacity retention after 200 cycles (FIG. 12B). Further increasing to 4.4 V, 196.8 mAh/g discharge capacity was seen (FIG. 12C) along with a 72.0% capacity retention after 200 cycles. Note that 200 cycles at C/10 charge rate and C/3 discharge rate mean 2600 hours of cycling. The total testing time was equal to a cell undergoing 1300 cycles at 1C. Increased polarization (FIGS. 13A-13C) was observed when the cutoff voltage increased which is presumably assigned to intensified electrolyte decomposition at elevated voltages (FIGS. 14A-B, 15A-B, 16A-B) and thus higher impedance resulting from cathode passivation films and single crystal lattice change. Crystalline gliding and cracking were seen when the cutoff voltage was beyond 4.3V (FIGS. 13A-130). Table 4 summarizes the electrochemical performances and testing conditions of all previously published single crystalline Ni-rich NMC (Ni>0.6) cathode materials.

TABLE 3 Cell design parameters for 250 Wh/kg lithium ion pouch cell based on graphite/NMC76 chemistry Material LiNi_(0.76)Mn_(0.14)Mn_(0.10)O₂ Cathode 1^(st) discharge capacity/ 200 mAh g⁻¹ Active material loading 96% Cathode weight (each 21 side)/mg cm⁻² Areal capacity (each 4.0 side)/mAh cm⁻² Electrode press density/ 3.0 g cm⁻³ Electrode thickness 70 (each side)/μm Number of double side 13 layers Electrode dimension 36 * 54 W * L/mm Al foil Thickness/μm 12 Anode Material Graphite Specific capacity/ 360 mAh g⁻¹ Active material loading 96% N/P ratio (cell balance) 1.16 Coating weight (each 13.5 side)/mg cm⁻² Electrode dimension 37.5 * 55.5 W * L/mm Cu foil Thickness/μm 8 Electrolyte E/C ratio/g Ah⁻¹ 2.5 Separator Thickness/μm 20 Packaging Thickness/μm 88 foil Cell Average voltage 3.65 (1^(st) cycle)/V Capacity (1^(st) cycle)/Ah 2.0 Cell energy density/ 250 Wh kg⁻¹

TABLE 4 Summary of single crystalline Ni-rich NMC (Ni > 0.6) reported in literature Cycling Initial Capacity Total Capacity Voltage Retention cycling Ref. Composition (mAh/g) Rate (V) Loading No. (%) Rate time (hr)* This LiNi_(0.76)M_(0.14)C_(0.1)O₂ 196.8 0.1 C 4.4 vs. Gr 20 mg/cm² 200 73 0.1/0.33 C 2389 work (full cell) 1 LiNi_(0.8)M_(0.1)C_(0.1)O₂ 185 0.1 C 4.2 vs. Li 3 mg/cm²  25 50 0.1/0.1 C ~500 LiNi_(0.5)M_(0.1)C_(0.1)O₂ 240 0.1 C 4.6 vs. Li —  25 50 0.1/0.1 C 2 LiNi_(0.88)M_(0.09)C_(0.08)O₂ 192 0.2 C 4.3 vs. Li 12 mg/cm² 100 88 0.2/0.2 C ~1000 (full cell) 3 LiNi_(0.8)M_(0.1)C_(0.1)O₂ 190 0.1 C 4.3 vs. Li 3 mAh/cm² 100 ~90 1/1 C ~200 4 LiNi_(0.83)M_(0.08)C_(0.11)O₂ 184.1   1 C 4.2 vs 47 mg/cm² 600 84.8 1/1 C ~1200 Gr/SiO (tested at 45° C., full cell)  5** LiNi_(0.92)M_(0.01)C_(0.06)AlW_(x)Mo_(x)O₂ 221.4 0.1 C 4.3 vs. Li 7.8 mg/cm² 100 95.7 0.5/1 C 100 *Testing time of single crystalline LiNi_(0.76)M_(0.14)C_(0.1)O₂is captured from testing data. Testing time for reference results are evaluated according to the current density. **x corresponds to 1000 ppm for W and Mo 1Zhu et al., J Mater Chem A 2019, 7: 5483-5474; 2Li et al., J Electrochem Soc 2019, 166: A1956-A1963; 3Qian et al., Energy Storage Mater 2020, 27: 140-159; 4Fan et al., Nano Energy 2020, 70: 104450; 5Yan et al., J Electrochem Soc 2020, 167: 120514.

Lattice gliding was clearly observed in single crystalline NMC76 at high voltages. Between 2.7 and 4.2 V (vs. graphite), the entire single crystal was well maintained after 200 cycles (FIG. 13A). Increasing cutoff voltage to 4.3 V, there were some gliding lines seen on the crystal surfaces after 200 cycles (FIG. 13B). Cycled to 4.4 V, single crystals appeared to be “sliced” (FIGS. 13C, 17A-17F) in parallel, along the (003) plane and vertical to c-axis of the layered structure (FIG. 18C), which indicated a model II type crack (in-plane shear) in fracture mechanics. Additionally, small cracks that indicate a model I type fracture (opening) were also discovered at 4.4 V (FIG. 13C). All characterizations were done by selecting various regions of NMC76 electrodes and the same phenomenon was repeatedly found (FIGS. 19-21). Although single crystalline NMC76 as a whole particle was still intact (FIG. 12A), gliding was the major mechanical degradation mode especially when cutoff voltage is above 4.3 V. Of note, the “gliding steps” formed in cycled crystals are quite different from cracking along intergranular boundaries of polycrystalline NMC particles. The scanning transmission electron microscopy (STEM) image for single crystalline NMC76 (FIGS. 18B-18D) confirmed that on both sides of a gliding plane (yellow line in FIG. 18D), the d-spacing of (003) plane (0.48 nm) was unchanged and the layered structure was well maintained after the “gliding” marks occurred. The long-range lattice symmetry of the bulk material will thereby not be altered. Ni, Mn, Co and O were still uniformly distributed in the vicinity of glided planes based on electron energy loss spectroscopy (EELS) analysis (FIG. 18F and FIG. 22). The uniform element distribution and intimately attached lattices across the gliding planes strongly demonstrated that although planar gliding occurs, no new boundary was generated, and the “sliced” area maintained the same lattice structure and chemical conditions as in the bulk phase. It should be noted that the gliding line (or the slicing marks) cannot be observed on the cross section of bulk particles by SEM, and are only visible by STEM bright field (BF) on thin sliced TEM samples. Although the internal lattice symmetry was well maintained after the gliding, the repeated gliding near surface eventually will evolve into microcracks exposing new surfaces to the electrolyte (FIGS. 13A-13C).

To further induce lattice gliding, the cutoff voltage of NMC76 single crystal is raised to 4.8 V (vs. Li+/Li). “Slicing marks” and microcracks are present in almost every charged single crystal (FIG. 23A). Slight deformation of individual single crystals is clearly observed (FIG. 18G), probably because the gliding of each layer equally likely moves towards symmetrically equivalent directions. Surprisingly, after discharging back to 2.7 V, the majority of single crystals revert to their original morphologies and the previously observed steps and microcracks disappear (FIG. 23B). The “glided” layers within single crystals almost completely “glided” back to their original locations (FIG. 18H), fully recovering from the deformation (FIG. 18G), although some “traces” are visible (labeled in FIG. 18H). Within the “regular” electrochemical window of 2.7-4.4 V (vs. graphite), after extensive cycling, lattice gliding and microcracking are also seen within crystal lattice at charged status (FIG. 18I). STEM analysis of the NMC76 crystal (FIG. 18J) indicates that the microcracks initiate from inside of the crystal. At the discharge status of those cycled crystals (cut off at 4.4 V), few ridges or cracks are found on the crystals. Although not as visible as in charged crystals, STEM still uncovers some “slicing marks” (FIG. 18K) on discharged single crystal NMC76 which probably undergoes reversible “sliding” process back and forth during 120 cycles. No microcracking is identified in those “self-healed” single crystals (FIG. 18L), suggesting that the lattice gliding and cracking in some of the crystals are still reversible after 120 cycles. As cycling continues, particle deformation will become dominant. Dislocation was also observed near the tip regions of microcrack of single crystals charged at 4.4 V (FIGS. 24A-24D). The accumulation of dislocation was accompanied by the microcrack propagation. A trace amount of nano-sized NiO-like rock salt phase (FIGS. 25A-25E) was observed on the gliding exposure step area of single crystalline NMC76 after cycling.

In situ AFM has been used to image the crystal surface in real time in an electrochemical cell. A ˜3 μm sized NMC76 single crystal was studied by in situ AFM during charge and discharge (FIGS. 26A-26F). Regions B and C in FIG. 26A are enlarged in FIGS. 26B and 26C, respectively, to probe the origin and evolution of “gliding steps” and microcracks under the electrical field. The formation of nanosized crack domains was observed on the side surface from open circuit voltage (OCV) to 4.50 V (vs. Li+/Li) during charge, while these domains disappeared in the discharge process (FIG. 26B). Moreover, planar gliding was characterized by the appearance of wide crystal steps on the side surface due to the uneven movement between neighboring layers during polarization. More wide gliding steps were observed on the side surface starting at 4.20 V charging process and led to the more and wider (˜85 nm) gliding steps at 4.50 V (FIG. 26C). When the cell potential decreased to 4.19 V, a few wide gliding steps decreased in their width (FIGS. 27A-27B), indicating the atomic layer recovered back to their original position (FIG. 26C). The average width of the steps almost linearly increased with increasing voltages (vs. Li+/Li) during the charging process followed by linear decreases with decreasing voltages during the discharge process, with its value of 30.3±0.7 nm at open circuit voltage (OCV), up to 52.5±2.5 nm at 4.50 V at charge status, and down to 38.4±1.3 nm at 4.19 V during discharge. The average step width was calculated by dividing the total width of the lateral face by the total number of steps (between 21 and 43 steps) for each voltage during the in situ AFM monitoring. The error bars in the step width were evaluated by averaging of three times of measurement of lateral face width at each time point. This “first increase then decrease” behavior of average step width vs. voltage indicates the reversible gliding process of these NMC crystals in each cycle. The reversible gliding process is further illustrated in FIG. 26F. The observed lattice gliding is a direct observation of the “Lattice-Invariant Shear (LIS)” (Radin et al., Nano Lett 2017, 17:7789-7795). LIS should exist in many layered electrode materials, which experience stacking-sequence-change phase transformations due to lithium concentration change. It was also predicted that LIS will lead to particle deformation and ridges on the particle surface, but these signals are likely to be buried in the internal boundaries in a spherical-secondary polycrystalline. The micron-sized single crystal provides a clear platform to observe gliding or LIS induced mechanical degradation.

Electrochemical potential difference is the driving force of lithium-ion diffusion and the formation of the lithium concentration gradient (Xiao, Sci 2019, 366:426-427). Stress will be generated during Li+ diffusion after establishing a lithium concentration gradient in the lattice. An analytical cylindrical isotropic diffusion-induced stress model was applied to understand the stress generation when lithium ions diffuse along the radial direction in the particle. The analytical solution of the dimensionless principal tress along axial, tangential and radial directions inside this cylindrical particle experienced compression or tension forces during cycling and reached maximum stresses when the Li+ concentration gradient was the highest. The peak tensile stress along tangential and axial directions occurred near the surface at the onset of delithiation (or 0.01T) (FIGS. 28A-28D). Conversely, the peak tensile stress in all three directions occurred at the center of the particle (FIGS. 29A-29D) during lithiation. During charge (delithiation), the tensile stress along axial and tangential directions was localized on the surfaces of single crystals, leading to microcrack opening normal to (003) planes. FIGS. 30A-30C are SEM images showing microcracks, which propagate from the center to the surface, forming fractures.

Local stress also has a shear component along other directions, which is solved numerically via COMSOL. The shear stress component along yz direction that can trigger the gliding along the (003) planes is shown in FIGS. 20D and 20E. Although the signs of the shear stress during lithiation and delithiation are opposite, which explains the reversible gliding, the absolute values are not the same (FIGS. 31A-31D), since the elastic modulus is a function of Li concentration. Therefore, the gliding motion should be largely but not completely reversible. The peak stresses in FIGS. 31C and 31D are not exactly the same which provides the explanation of the largely but not completely reversible gliding in single crystalline NMC76 particles. Comparing the stress difference between FIGS. 31C and 31D, the anisotropic volume expansion (chemical strain) will lead to increased shear stress. The peak shear stresses are inside the particle during lithiation and delithiation, suggesting the sliding is likely to initiate inside of the particles. The irreversible gliding can generate small damages, being accumulated into the crack opening over long time cycling, an analog of fatigue crack nucleation. These lead to the ridges and microcracks seen on the surfaces of single crystals after cycling.

The simple isotropic diffusion-induced-stress model can be used to predict if the cracks can be stabilized inside of the single crystal. Since the strain energy inside the particle reaches a maximum around the scaled time of Tp=0.1T during delithiation (FIGS. 22A-22D), its comparison with the fracture energy (2γ) is used as a criterion to evaluate the critical size of single crystal NMC76. If the accumulated strain energy is not large enough to cleave entire the crystal, the crack will be stabilized inside of the particle.

$\begin{matrix} {{\Pi ❘_{T_{p}}} = {{\int{\frac{\sigma^{2}}{2E}{dV}}} = {{\pi*h*\left\lbrack \frac{a*E_{0}*\left( {C_{R} - C_{0}} \right)}{1 - v} \right\rbrack^{2}*{\int_{0}^{r}{\xi^{2}\frac{1}{E}{rdr}}}} < {2\gamma}}}} & (1) \end{matrix}$

where h is the height of the cylindrical particle, α is the concentration expansion coefficient, E₀ is Young's modulus of the nonlithiated particle, E is Young's modulus at a given lithium-ion concentration, C_(R) is the lithium-ion concentration at the surface, C₀ is the lithium-ion concentration at the center, vis Poisson's ratio and ξ represents the dimensionless stress (FIGS. 22A-D, 23A-D), and γ is the surface energy. A lower bound estimation of the critical size of the single crystal is predicted to be ˜3.5 μm, below which cracks can be considered stable inside of the particle. The simulation result suggests that although fractures along (003) direction appear in single crystals during cycling, the cracks are stable once formed and will not initiate catastrophic reactions to produce a fracture zone that eventually pulverizes the entire single crystal. Increasing the applied current density will lead to higher concentration gradient and higher stress generation. Increasing the cutoff voltage is equivalent to increasing (C_(R)−C₀) in equation (1). It means higher stress generation and large strain energies at elevated voltages, which causes more “gliding” and “cracking” (FIGS. 7A-7C). The findings provide some strategies to stabilize single crystalline Ni-rich NMC by either reducing the crystal size to below 3.5 μm, absorbing accumulated strain energy through modification of the structure symmetry, or simply optimizing the depth of charge without sacrificing much reversible capacity.

FIG. 32 shows an SEM image of a 20 μm single crystal NMC76 and images obtained by in situ AFM. Selective formation of a passivation film on certain planes of the NMC76 was found. Dissolution and recrystallization were seen on surfaces of the single crystal. Parallel cracking along the [001] surface was directly captured.

Example 2 Molten-Salt Synthesis and Characterization of Single Crystal LiNi_(0.76)M_(0.14)C_(0.1)O₂ (NMC76) Synthesis

A hydroxide precursor was prepared as follows. A 2 M solution of transition metal (TM) sulfate solution (Ni:Mn:Co=0.76:0.14:0.10 in molar ratio) was prepared in 805 g deionized water Ni(SO₄)₂.6H₂O, MnSO₄.H₂O, and Co(SO₄)₂.7H₂O. Separately 160 g NaOH was dissolved in 500 g H₂O. Concentrated NH₃.H₂O (28%) was diluted using DI H₂O at 1:1 volume ratio. 1.5 L DI H₂O and 50 mL concentrated NH₃.H₂O (28%) were added into the reactor as the starting solution. The reactor was heated to 50° C. The TMSO₄ solution, NaOH and NH₃.H₂O were pumped into the reactor at same time. The pump rates were 3 mL/min and 1 mL/min for TMSO₄ and NH₃.H₂O, respectively. The pH was controlled at 11.0-11.5. When all TMSO₄ solution was added into the reactor, the co-precipitated hydroxides were aged in the reactor for 30 hrs. The precipitates were filtered and washed with DI H₂O. 200 g DI H₂O was used for every 100 g precipitates in washing for 3-5 times. After drying at 100° C. for 12 hours, the hydroxide precursor Ni_(0.76)Mn_(0.14)Co_(0.1)(OH)₂ was obtained. Each batch produced ˜150 g of the hydroxide precursor.

The mixed hydroxide precursor was heated in air for 15 hours to decompose into mixed oxides. Three different temperatures, 800, 900, and 1000° C., were used to study the influence of temperature on the oxide particle size. The ramping rate was 5° C./minute. The morphology of the pristine hydroxide precursor and the oxides obtained at 800, 900, and 1000° C. are shown in FIGS. 33A-33D, respectively. The optimal calcination temperature was found to be 900° C. for Ni_(0.76)Mn_(0.14)Co_(0.1)(OH)₂. Suitable temperatures range from 400-1000° C.

The oxide precursors were mixed with Li₂O at 1:0.6 molar ratio (TM:Li=1:1.2), then the TM-Li mixture was mixed with sintering agent NaCl using a 1:1 weight ratio. The TM-Li—NaCl mixture was heated in a tube furnace filled by flowing pure oxygen gas. The temperature was increased at 10° C./min ramping rate. The mixture was maintained at 800° C. for 10 hours, then at 900° C. for an additional 5 hours. The product was cooled to room temperature. The sintered product (brick) was ground with an agate mortar and then transferred to a beaker for washing away NaCl. For each 15 g of sample, 30 g of water was added. The ground sample was stirred in water for two minutes. After ultrasonication for 2 minutes, the mixture was stirred for an additional 10 minutes to dissolve all residual NaCl. After filtration, the washed powders were heated at 80° C. in vacuum for 2 hours to remove water. The dried samples were further sintered at 580° C. in pure oxygen for four hours to restore some lost oxygen in the lattices.

A schematic diagram of the process is shown in FIG. 34. FIG. 34 also shows SEM images of the hydroxide precursor, the oxide precursor, and the single crystal product.

In the absence of NaCl as a sintering agent, agglomerated of LiNi_(0.76)Mn_(0.14)Co_(0.1)O₂ (NMC76) particles are formed instead of single crystals. NaCl is an inexpensive sintering agent, lowering the cost for scaling up the synthesis. FIGS. 35A-35B are SEM images of NMC76 prepared without NaCl sintering agent (FIG. 35A) and with NaCl (FIG. 35B). Without NaCl, the sample was polycrystalline (FIG. 35A). With NaCl, the sample was monocrystalline (FIG. 35B).

FIG. 36A shows the initial charge-discharge curve of NMC76 prepared with and without NaCl. FIG. 36B shows the cycling stability of a thick single crystal NMC76 electrode (20 mg/cm²) in a full cell using graphite as the anode between 2.7-4.2V, charge at 0.1C and discharge at 0.33C. 1C=200 mA/g. FIG. 36C shows the cycling stability of a single crystal NMC76 electrode (21.5 mg/cm²) in a full cell using graphite as the anode between 2.7-4.3V.

Additional samples were prepared to compare washing with water and other solvents. The samples washed with deionized water displayed the best structural integrity and best electrochemical performance. FIGS. 37A and 37B are SEM images of single crystal LiNi_(0.7)Mn_(0.22)Co_(0.08)O₂ washed with water (37A) or formamide (FM) (37B). FIG. 37C shows the initial charge-discharge curves of the two samples.

FIG. 38 is a schematic diagram comparing synthesis processes for polycrystalline and monocrystalline LiNixMn_(y)Co_(1−x−y)O₂. When synthesis proceeds directly from hydroxide precursors, a polycrystalline product is formed. However, when synthesis precedes via oxide precursors as described herein, single crystals are obtained.

Example 3 Flash-Sintering Synthesis and Characterization of Single Crystal LiNi_(0.76)M_(0.14)C_(0.1)O₂ (NMC76) Synthesis

Hydroxide precursors were prepared as described in Example 2. The hydroxide precursors and LiOH were mixed in a molar ratio of 1:1.2 and ground with an agate mortar. The mixture was transferred to a flash sintering furnace and heated to 800° C. in a pure oxygen atmosphere at ramping rates ranging from 2° C./minute to 20° C./minute. The temperature was maintained at 800° C. for 10 hours. No sintering agent was used. Ramping rates up to 50° C./second or faster may be used.

FIGS. 39A-39C are SEM images of LiNi_(0.76)Mn_(0.14)Co_(0.1)O₂ prepared at ramping rates of 2° C./min (39A), 10° C./min (39B), and 20° C./min (39C). As the ramping rate increased, the particle size increased. The agglomeration of particles was also significantly reduced when heating rate was increased at 20° C., leading to the formation of large single crystals (FIG. 39C). At a ramping rate of 2° C., however, the particles are mostly aggregated together forming secondary particles instead of individual single crystals.

A preheating process was found to facilitate flash sintering. Without wishing to be bound by a particular theory of operation, preheating reduces mismatch of reaction rates. The hydroxide precursor and lithium hydroxide mixtures were pre-heated at 480° C. for 5 hours before flash sintering. During this pre-heating treatment, mixed oxides (including Li oxide) formed. It was also found that the NMC single crystals derived from preheated precursors demonstrated smaller particle sizes which improved the electrochemical kinetics of single crystal NMC. FIGS. 40A and 40B are SEM images of LiNi_(0.76)Mn_(0.14)Co_(0.1)O₂ prepared without preheating (40A) or with preheating (40B) prior to flash sintering at a ramping rate of 50° C./minute. FIG. 41 shows the charge-discharge curves of the single crystal NMC samples prepared with and without the preheating process, showing a clear improvement in the reversible capacity with preheating.

Example 4 Solid-State Synthesis and Characterization of Single Crystal LiNi_(0.76)M_(0.14)C_(0.1)O₂ (NMC76) Synthesis

A hydroxide precursor was prepared as follows. A 2 M solution of transition metal (TM) sulfate solution (Ni:Mn:Co=0.76:0.14:0.10 in molar ratio) was prepared in 805 g deionized water using Ni(SO₄)₂.6H₂O, MnSO₄.H₂O, and Co(SO₄)₂.7H₂O. Separately 160 g NaOH was dissolved in 500 g H₂O. Concentrated NH₃.H₂O (28%) was diluted using DI H₂O at 1:1 volume ratio. 3 L DI H₂O and 50 mL concentrated NH₃.H₂O (28%) were added into the reactor as the starting solution. The reactor was heated to 50° C. The TMSO₄ solution, NaOH and NH₃.H₂O were pumped into the reactor at same time. The pump rates were 3 mL/min and 1 mL/min for TMSO₄ and NH₃.H₂O, respectively. The pH was controlled at 10.8. When all TMSO₄ solution was added into the reactor, the co-precipitated hydroxides were aged in the reactor for 30 hrs. The precipitates were filtered and washed with DI H₂O. 200 g DI H₂O was used for every 100 g precipitates in washing for 3-5 times. After drying at 100° C. for 12 hours, the hydroxide precursor Ni_(0.76)Mn_(0.14)Co_(0.1)(OH)₂ was obtained. Each batch produced ˜150 g of the hydroxide precursor.

The Ni_(0.76)Mn_(0.14)Co_(0.1)(OH)₂ was heated at 900° C. for 15 hours in an oxygen atmosphere with a 10° C./min ramping rate. The hydroxide precursors were converted to oxide precursors in this step.

The oxide precursors were mixed with LiOH at a Li:TM molar ratio of 1:1.07 and annealed at 500° C. for 5 hours. The product was cooled to room temperature and ground. A second annealing was performed at 800° C. for 5 hours. After grinding again at room temperature, a third annealing was performed at 800° C. for an additional 5 hours. The final product was passed through a 400-mesh sieve and collected.

FIGS. 42A and 42B are SEM images of the small particles of Ni_(0.76)Mn_(0.14)Co_(0.1)(OH)₂ and the Ni_(0.76)Mn_(0.14)Co_(0.1)O₂ precursors, respectively. FIG. 42C is an SEM image of the single crystal LiNi_(0.76)Mn_(0.14)Co_(0.1)O₂. Use of small hydroxide precursor particles facilitates synthesis of the desired monocrystalline LiNi_(0.76)Mn_(0.14)Co_(0.1)O₂. FIG. 43 shows the first charge and discharge curve of the LiNi_(0.76)Mn_(0.14)Co_(0.1)O₂ at 0.1C between 2.7-4.4 V.

Cathodes comprising the monocrystalline LiNi_(0.76)Mn_(0.14)Co_(0.1)O₂ prepared by the molten salt method of Example 2 and the monocrystalline LiNi_(0.76)Mn_(0.14)Co_(0.1)O₂ prepared by the solid-state method were compared. FIGS. 44A and 45A show the charge-discharge curves of the two cathodes. The monocrystalline LiNi_(0.76)Mn_(0.14)Co_(0.1)O₂ prepared by the solid-state method delivered ˜184 mAh/g. It is expected that further development will provide a reversible capacity of NMC811 single crystals of >200 mAh/g, competitive with commercially available polycrystalline NMC, but with greatly enhance stability and safety. FIGS. 44B and 45B are SEM images of the monocrystalline LiNi_(0.76)Mn_(0.14)Co_(0.1)O₂ used in each cathode. The individual crystal size in FIG. 44B is ˜3 μm. The individual crystal size in FIG. 45B is ˜1 μm.

Example 5 Synthesis and Characterization of Single Crystal LiNi_(0.76)M_(0.12)C_(0.1)Mg_(0.01)Ti_(0.01)O₂ Synthesis

A hydroxide precursor was prepared as follows. A 2 M solution of transition metal (TM) sulfate solution (Ni:Mn:Co:Mg:Ti=0.76:0.12:0.10:0.01:0.01 in molar ratio) was prepared in 805 g deionized water using Ni(SO₄)₂.6H₂O, MnSO₄.H₂O, Co(SO₄)₂.7H₂O, MgSO₄, and TiOSO₄. Separately 160 g NaOH was dissolved in 400 g H₂O. Concentrated NH₃.H₂O (28%) was diluted using DI H₂O at 1:1 volume ratio. 1.5 L DI H₂O and 50 mL concentrated NH₃.H₂O (28%) were added into the reactor as the starting solution and preheated to 50° C. The TMSO₄ solution, NaOH and NH₃.H₂O were pumped into the reactor at same time. The pump rates were 3 mL/min and 1 mL/min for TMSO₄ and NH₃.H₂O, respectively. The pH was controlled at 11.5. When all TMSO₄ solution was added into the reactor, the co-precipitated hydroxides were aged in the reactor for 30 hrs at 50° C. The precipitates were filtered and washed with DI H₂O. 200 g DI H₂O was used for every 100 g precipitates in washing for 3-5 times. After drying at 100° C. for 12 hours, Mg—Ti-doped hydroxide precursors were obtained.

The doped hydroxide precursors were heated at 900° C. for 15 hours in an oxygen atmosphere with a 10° C./minute ramping rate. The hydroxide precursors were converted to oxide precursors in this step.

The oxide precursors were mixed with Li₂O (1:1.4 molar ratio). NaCl was then added in a NaCl:TM-Li mixture of 1:1.1 by weight. The resulting mixture was then annealed at 800° C. for 10 hours and then 900° C. for 5 hours. The product was cooled to room temperature. The sintered product was ground with an agate mortar and then transferred to a beaker for washing away NaCl. For each 15 g of sample, 30 g of water was added. The ground sample was stirred in water for two minutes. After ultrasonication for 2 minutes, the mixture was stirred for an additional 10 minutes to dissolve all residual NaCl. After filtration, the washed powders were heated at 80° C. in vacuum for 2 hours to remove water. The dried samples were further sintered at 580° C. in pure oxygen for four hours to restore some lost oxygen in the lattices and provide LiNi_(0.76)Mn_(0.12)Co_(0.1)Mg_(0.01)Ti_(0.01)O₂, which includes 1 at % Mg and 1 at % Ti.

The modified single crystal has a slightly reduced particle size at ca. 2 μm with a very dense structure (FIG. 46). The modified single crystal has a reduced peak ration of (003)/(104), suggesting increased cation disorder (FIG. 47A). Peak shifting to a lower angle in the modified single crystal indicates expansion of the crystal lattice compared to pristine single crystal NMC76 (47B).

FIGS. 48A and 48B compare the charge-discharge curves (48A) and cycling stability (48B) of Li Ni_(0.76)Mn_(0.14)Co_(0.1)O₂ and LiNi_(0.76)Mn_(0.12)Co_(0.1)Mg_(0.01)Ti_(0.01)O₂. The modified single crystal NMC76 delivered at slightly slower capacity at ca. 191 mAh/g capacity compared to the pristine single crystal NMC76 in a full cell tested at relevant conditions—high mass loading and thick electrodes. The modified NMC76 displayed improved cycling stability with 81.8% capacity retention after 200 cycles, compared to pristine NMC76 with 72.0% capacity retention. As shown in FIG. 49, no obvious cracking was observed in the modified NMC76 after cycling.

Example 6 Synthesis and Characterization of NMC from Li₂O and Oxide Precursors

NMC was prepared from LiOH and transition metal hydroxide precursors (TMOH) and compared to NMC prepared from Li₂O and transition metal oxide precursors (TMO).

Product 1—hydroxide precursors. LiOH and TMOH were mixed by a roller mixer (1.3:1 molar ratio). The mixture was first calcined at 500° C. for 5 hours in O₂ atmosphere. The product was then milled and calcined at 800° C. for 5 hours in O₂ atmosphere. The product was milled again and calcined at 800° C. for 5 hours in O₂ atmosphere. The product was milled again and sieved. Finally, the product was washed by water, dried in vacuum oven and then calcined 580° C. for 4 hours in O₂ atmosphere.

Product 2—oxide precursors with milling after first calcination. Li₂O was prepared by milling Li₂O granules to <150 um. Transition metal oxide (TMO) was prepared by decomposition of transition metal hydroxides (TMOH) prepared by a coprecipitation method, as described in the foregoing examples. The Li₂O and TMO were mixed (1.3:1 molar ratio between Li and transition metal). The mixture was first calcined at 800° C. for 10 hours in O₂ atmosphere. Then the product was milled and sieved. Finally, the product was washed by water, dried in vacuum oven and then calcined at 580° C. for 4 hours in an O₂ atmosphere.

Product 3—oxide precursors with no milling after calcination. Li₂O was prepared by milling Li₂O granules to <150 um. Transition metal oxide (TMO) was prepared by decomposition of transition metal hydroxides (TMOH) prepared by a coprecipitation method, as described in the foregoing examples. The Li₂O and TMO were mixed (1.3:1 molar ratio between Li and transition metal). The mixture was first calcined at 800° C. for 10 hours in O₂ atmosphere. Then the product was washed by water, dried in vacuum oven and then calcined at 580° C. for 4 hours in an O₂ atmosphere.

Cathodes were prepared from each of the products and evaluated. FIG. 50 shows the first cycle voltage profiles of each product. Using Li₂O as the starting material resulted in similar first cycle charge and discharge capacities compared to Product 1 prepared from LiOH and TMOH. FIG. 51 is SEM images of Products 2 and 3. As shown in FIG. 51, similar particles were observed with (left image) and without (right image) milling and sieving, proving the particles do not agglomerate to form chucks and milling or sieving is thus not needed when Li₂O is used as the lithium source.

Example 7 Synthesis and Characterization of Monocrystalline NMC811 from Li₂O and Oxide Precursors

Mixtures of Li₂O and TMO were prepared (1.3:1 molar ratio between Li and transition metal). The mixtures were calcined at temperatures ranging from 750° C. to 950° C. for 10 hours in O₂ atmosphere. Then the product was washed by water, dried in vacuum oven and then calcined at 580° C. for 4 hours in an O₂ atmosphere.

FIG. 52 shows SEM images of the TMO precursor, TMO mixed with unmilled Li₂O, and NMC811 prepared at 750° C., 800° C., 850° C., 900° C., 925° C., and 950° C. Previously, 800° C. was determined to be an optimal temperature for preparing NMC from LiOH. However, Li₂O-derived crystals prepared at 750 and 800° C. were underdeveloped (second and third upper images). However, when the temperature was 900° C. or greater, single crystal growth was accelerated (second through fourth lower images). At 950° C. (lower right image), most particles were individual micron-sized single crystals without any aggregations; all secondary particles were sintered into a single crystal. FIGS. 53A-53C show that the use of Li₂O leads to uniform particles without tiny flakes.

Two half cells were prepared for each product with an electrolyte comprising 1 M LiPF₆ in EC-EMC (3:7 wt)+2 wt % VC. The electrodes comprised 8 parts NMC811, 1 part carbon additives, and 1 part PVDF binder. The cells were cycled from 2.7-4.4 V, 3 formation cycles at 0.1C/0.1C with cutoff voltage (CV) to C/20, then 0.2 C to 0.33C with CV to C/20. FIG. 54 shows the first cycle charge and discharge capacity of NMC811 prepared from LiOH (800° C.) or Li₂O (850° C.). The data is summarized below in Table 5.

TABLE 5 1^(st) delithiation 1^(st) lithiation Salt capacity capacity (cell 1/cell 2) (mAh/g) (mAh/g) 1^(st) CE Li₂O 226/226 201/202 89.0%/89.3% LiOH · H2O 224/222 197/194 87.7%/87.3%

FIG. 55 shows capacity of the cells over 20 cycles. The NMC prepared from Li₂O provided similar cycling capacity to the baseline NMC prepared from LiOH.H₂O.

FIGS. 56A and 56B show slurry properties of NMC811 prepared from Li₂O and other salts. The NMC811 is mixed with PVDF (polyvinylidene fluoride) and carbon additives in NMP (N methyl 2 pyriolidone) solvent to form a slurry. While the slurry of NMC811 prepared from other salts, e.g. LiOH undergoes gelation and behaves like a solid and cannot be applied as a coating on a current collector to form an electrode (FIG. 56A), the slurry of NMC811 prepared from Li₂O is in free flowing state and can be readily coated on a current collector to form a uniform electrode (FIG. 56B).

Example 8 Pouch Cell Design

Table 6 provides parameters for 2.2-2.3 Ah pouch cells using graphite/NMC811 and graphite/NMC955 chemistries. Coin cells with similar cathode loading, areal capacity, porosity, press density, N/P ratio, and the like, will be used for initial testing. Replacing graphite with Si or Li metal may increase the cell level energy to 300-350 Wh/kg with the single crystal NMC cathode.

TABLE 6 Cell design parameters for 2.2-2.3 Ah pouch cell designs based on graphite/NMC chemistry Material NMC811 NMC955 Cathode 1^(st) discharge capacity 200 210 (mAh g⁻¹⁾ Active material loading  96%  96% Cathode weight (each side) 18.3 18.3 (mg cm⁻²) Areal capacity (each side) 3.5 3.7 (mAh cm⁻²) Electrode press density 3.0 3.0 (g cm⁻³) Electrode thickness (each 61 61 side) (μm) Number of double side 16 16 layers* Al foil Thickness (μm) 10 10 Anode Material graphite graphite Specific capacity (mAh g⁻¹) 360 360 Active material loading  96%  96% Coating weight (each side) 11.4 12.0 (mg cm⁻²) Areal capacity (each side) 3.9 4.1 (mAh cm⁻²) N/P ratio (cell balance) (μm) 1.12 1.12 Cu foil Thickness/μm 8 8 Electro- Electrolyte/Capacity ratio 3.5 3.4 lyte (g Ah⁻¹) Separator Thickness/μm 20 20 Packet Thickness/μm 88 88 foil Cell Average voltage (1^(st) cycle) 3.65 3.7 (V) Capacity (1^(st) cycle) (Ah) 2.2 2.3 Cell energy density 250 264 (Wh kg⁻¹) *number of cathodes comprising Al foil sandwiched between two NMC coating layers

In view of the many possible embodiments to which the principles of the disclosed invention may be applied, it should be recognized that the illustrated embodiments are only preferred examples of the invention and should not be taken as limiting the scope of the invention. Rather, the scope of the invention is defined by the following claims. We therefore claim as our invention all that comes within the scope and spirit of these claims. 

We claim:
 1. A method comprising: making lithium nickel manganese cobalt oxide by combining a molar excess of Li₂O with a solid oxide precursor comprising NixMn_(y)M_(z)Co_(1−x−y−z)O₂, where M represents one or more dopant metals, x≥0.6, 0.01≤y<0.2, 0≤z≤0.05, and x+y+z≤1.0; heating the solid oxide precursor and the Li₂O in an oxygen atmosphere at a first temperature T₁ for a first effective period of time t₁ to produce a first product; cooling the first product; and heating the first product in an oxygen atmosphere at a second temperature T₂ for a second effective period of time t₂ to produce lithium nickel manganese cobalt oxide having a formula LiNixMn_(y)M_(z)Co_(1−x−y−z)O₂.
 2. The method of claim 1, wherein the Li₂O has a mean particle size of less than or equal to 150 μm.
 3. The method of claim 1, wherein the dopant metal M, if present, comprises Mg, Ti, Al, Zn, Fe, Zr, Sn, Sc, V, Cr, Fe, Cu, Zu, Ga, Y, Zr, Nb, Mo, Ru, Ta, W, Ir, or any combination thereof.
 4. The method of claim 1, wherein z is 0, and the lithium nickel manganese cobalt oxide has a formula LiNixMn_(y)Co_(1−x−y)O₂.
 5. The method of claim 1, wherein the first product is not milled or sieved prior to heating the first product in the oxygen atmosphere.
 6. The method of claim 1, wherein: (i) the first temperature T₁ is 800° C. to 1000° C.; or (ii) the first effective period of time t₁ is 1 hour to 30 hours; or (iii) the second temperature T₂ is 500° C. to 800° C.; or (iv) the second effective period of time t₂ is 1 hour to 25 hours; or (v) any combination of (i), (ii), (iii), and (iv).
 7. The method of claim 1, wherein: (i) the first temperature T₁ is 900° C. to 1000° C.; (ii) the effective period of time t₁ is 5 hours to 15 hours; or (iii) both (i) and (ii).
 8. The method of claim 1, wherein: (i) the first temperature T₁ is 950° C. to 1000° C.; (ii) the effective period of time t₁ is 8 hours to 12 hours; or (iii) both (i) and (ii).
 9. The method of claim 1, wherein: (i) the temperature T₂ is 550° C. to 650° C.; (ii) the effective period of time t₂ is 2 hours to 6 hours; or (iii) both (i) and (ii).
 10. The method of claim 1, wherein: x=0.65-0.9; y=0.05-0.2; z=0-0.02; and x+y+z=0.7-0.95.
 11. The method of claim 1, wherein: x=0.75-0.9; y=0.05-0.14; z=0-0.02; and x+y+z=0.8-0.98.
 12. The method of claim 1, wherein the solid oxide precursor and the Li₂O are combined in a Li:solid oxide precursor molar ratio of 1.01:1 to 3:1.
 13. The method of claim 1, further comprising: washing the first product to provide a washed product; and heating the washed product at a temperature of 60° C. to 120° C. for a time of 1 hour to 10 hours prior to heating the first product at the temperature T₂ for the effective period of time t₂.
 14. The method of claim 1, wherein the first temperature T₁ is 925° C. to 1000° C., and the lithium nickel manganese cobalt oxide is monocrystalline.
 15. The method of claim 1, further comprising preparing the solid oxide precursor by: preparing a 1.5-2.5 M solution comprising metal salts in water, the metal salts comprising a nickel (II) salt, a manganese (II) salt, a cobalt (II) salt, and optionally one or more dopant metal salts, wherein a mole fraction x of the nickel (II) salt in the solution is 0.6, a mole fraction y of the manganese (II) salt is 0.01≤y<0.2, a mole fraction z of the one or more dopant metal salts is 0≤z≤0.05, a mole fraction of the cobalt (II) salt is 1−x−y−z, and x+y+z 1.0; combining the solution comprising metal salts in water with aqueous NH₃ and aqueous NaOH or KOH to provide a combined solution having a pH of 10.5-12 and a combined metal salt concentration of 0.1 M to 3 M; aging the combined solution for 1 hour to 48 hours at a temperature of 25° C. to 80° C. to co-precipitate hydroxides of nickel, manganese, and cobalt to provide a solid hydroxide precursor comprising NixMn_(y)M_(z)Co_(1−x−y−z)(OH)₂; and heating the solid hydroxide precursor comprising NixMn_(y)M_(z)Co_(1−x−y−z)(OH)₂ at a temperature of 500° C. to 1000° C. in an oxygen-containing atmosphere for 1 hour to 30 hours to convert the solid hydroxide precursor to the solid oxide precursor.
 16. A method comprising: making monocrystalline lithium nickel manganese cobalt oxide by combining a molar excess of Li₂O with a solid oxide precursor comprising NixMn_(y)M_(z)Co_(1−x−y−z)O₂, where M represents one or more dopant metals, x≥0.6, 0.01≤y<0.2, 0≤z≤0.05, and x+y+z≤1.0; heating the solid oxide precursor and the Li₂O in an oxygen atmosphere at 800° C. to 1000° C. for 5 hours to 15 hours to produce a first product; cooling the first product to ambient temperature; and heating the first product in an oxygen atmosphere at 500° C. to 800° C. for 2 hours to 6 hours to produce lithium nickel manganese cobalt oxide having a formula LiNixMn_(y)M_(z)Co_(1−x−y−z)O₂.
 17. The method of claim 16, wherein the solid oxide precursor and the Li₂O are heated in the oxygen atmosphere at 800° C. to 950° C. for 8 hours to 12 hours.
 18. The method of claim 16, wherein z is 0, and the product is LiNixMn_(y)Co_(1−x−y)O₂.
 19. The method of claim 16, wherein: x=0.65-0.9; y=0.05-0.2; z=0-0.02; and x+y+z=0.7-0.95.
 20. Lithium nickel manganese cobalt oxide having a formula LiNixMn_(y)M_(z)Co_(1−x−y−z)O₂, wherein: M represents one or more dopant metals, x≥0.6, 0.01≤y<0.2, 0≤z≤0.05, and x+y+z≤1.0; and a slurry comprising the lithium nickel manganese cobalt oxide, a solvent, and a binder does not undergo gelation.
 21. Lithium nickel manganese cobalt oxide having a formula LiNixMn_(y)M_(z)Co_(1−x−y−z)O₂, wherein: M represents one or more dopant metals, x≥0.6, 0.01≤y<0.2, 0≤z≤0.05, and x+y+z≤1.0; and the lithium nickel manganese cobalt oxide is devoid of lithium salts capable of reacting with water, carbon dioxide, or water and carbon dioxide. 